Add 'blockdev-snapshot' command

This series adds a new 'blockdev-snapshot' command, that is similar to 'blockdev-snapshot-sync' but takes references to two existing block devices.

This finally applies (and works) cleanly on master. Max's patch to allow creating BlockDriverState trees without a BlockBackend is now available as be4b67bc7d99da26b7878f7f45370f50a3bd4af5.

Regards,

Berto

v8:
- Rebase on top of the current master
- Patch 4: state->new_bs->backing_hd is now state->new_bs->backing

v7: https://lists.gnu.org/archive/html/qemu-block/2015-10/msg00273.html
- Rebase on top of the current master. qmp_marshal_input_blockdev_snapshot is renamed to qmp_marshal_blockdev_snapshot in order to make it build.
- New patch to use bdrv_lookup_bs() instead of bdrv_find_node() in external_snapshot_prepare(). This way, if the user attempts to use blockdev-snapshot-sync using an existing device ID in the snapshot-node-name parameter, the code will detect the error before the new image is created.

v6: https://lists.gnu.org/archive/html/qemu-block/2015-09/msg00575.html
- Update documentation and parameter names following Eric's
suggestions: 'device' -> 'node', 'snapshot' -> 'overlay'.
- Rebased on top of Max's "BlockBackend and media" v5

v5: https://lists.gnu.org/archive/html/qemu-block/2015-09/msg00483.html
- Don't delete the 'backing' option if it contains something different from an empty string.
- Rebase on top of the current master.

v4: https://lists.gnu.org/archive/html/qemu-block/2015-09/msg00372.html
- Implement the support for 'backing': '', drop 'ignore-backing', and update iotest 085 accordingly.
- Include sample 'blockdev-add' call in the 'blockdev-snapshot' documentation.
- Clarify that the snapshot must not have a backing file in the BlockdevSnapshot documentation.
- Update error message ("...node name already existing" -> "...exists").

v3: https://lists.gnu.org/archive/html/qemu-block/2015-09/msg00272.html
- Add 'ignore-backing' field to BlockdevOptionsGenericCOWFormat. This allows opening images but not their backing images.
- Check for op blockers in the snapshot node and make sure that it doesn't have any backing image.
- Remove extra check for the existence of the snapshot node:
bdrv_open() already does that.
- Extend iotest 085 to add tests for 'blockdev-snapshot'.
- Replace local_err with errp in some places where the former is unnecessary.
- Update command description.
- Add 'since' tag to the 'blockdev-snapshot' field in TransactionAction.

v2: https://lists.gnu.org/archive/html/qemu-block/2015-09/msg00094.html
- Add 'blockdev-snapshot' command instead of allowing passing options to 'blockdev-snapshot-sync'.
- Rename BlockdevSnapshot to BlockdevSnapshotSync

v1: https://lists.gnu.org/archive/html/qemu-block/2015-08/msg00236.html

Alberto Garcia (5):
block: check for existing device IDs in external_snapshot_prepare()
block: rename BlockdevSnapshot to BlockdevSnapshotSync
block: support passing 'backing': '' to 'blockdev-add'
block: add a 'blockdev-snapshot' QMP command
block: add tests for the 'blockdev-snapshot' command

 block.c                    |   7 ++
 blockdev.c                 | 166 ++++++++++++++++++++++++++++-----------------
 qapi-schema.json           |   4 +-
 qapi/block-core.json       |  34 +++++++++-
 qmp-commands.hx            |  38 +++++++++++
 tests/qemu-iotests/085     | 102 ++++++++++++++++++++++++++--
 tests/qemu-iotests/085.out |  34 +++++++++-
 7 files changed, 312 insertions(+), 73 deletions(-)

2.6.1