Add subcommand compare for qemu-img

This is 8th version of patch adding compare subcommand that compares two images. Compare has following criteria:
- only data part is compared
- unallocated sectors are not read
- in case of different image size, exceeding part of bigger disk has to be zeroed/unallocated to compare rest
- qemu-img returns:
- 0 if images are identical
- 1 if images differ
- 2 on error

v8:
- minor grammar fixes (no behavior change)

v7:
- split patch into pieces
- Quiet mode added for all relevant subcommands
- check non-shared part of disk after shared one
- minor docummentation and naming fixes

v6:
- added handling -?, -h options for compare subcommand

v5 (only minor changes):
- removed redundant comment
- removed dead code (goto after help())
- set final total_sectors on first assignment

v4:
- Fixed various typos
- Added functions for empty sector check and sector-to-bytes offset conversion
- Fixed command-line parameters processing

v3:
- options -f/-F are orthogonal
- documentation updated to new syntax and behavior
- used byte offset instead of sector number for output

v2:
- changed option for second image format to -F
- changed handling of -f and -F [1]
- added strict mode (-s)
- added quiet mode (-q)
- improved output messages [2]
- rename variables for larger image handling
- added man page content

Miroslav Rezanina (4):
block: Add synchronous wrapper for bdrv_co_is_allocated_above
qemu-img: Add "Quiet mode" option
qemu-img: Add compare subcommand
Add qemu-img compare documentation

 block.c               |   51 ++++++-
 blockdev.c            |    6 +-
 include/block/block.h |    5 +-
 qemu-img-cmds.hx      |   34 +++--
 qemu-img.c            |  381 +++++++++++++++++++++++++++++++++++++++++++++----
 qemu-img.texi         |   35 +++++
 6 files changed, 461 insertions(+), 51 deletions(-)