some virtio-scsi tracing

Recently we had to track down a (remote) problem with virtio-scsi tape passthrough. Paolo asked me if I could turn my ad-hoc debug printf()s into trace events.

I tested the series in the following way:
- enabled the two "processing hungry" events in "trace-events",
- built & installed qemu,
- using libvirt, started a guest with a passthru USB flash drive,
- logged in to the guest,
- on the host, enabled all virtio_scsi_* events using "virsh qemu-monitor-command",
- in the guest:
- mounted the disk,
- listed the directory tree,
- unmounted the disk,
- on the host, physically disconnected the flash drive.

I'm attaching the trace output (stderr backend, saved by libvirt); you should be able to pair segments of the file with the above actions.

Laszlo Ersek (2):
qemu_hexstr(): hexdump a small buffer to a string, for in-line
printing
add some virtio-scsi trace events

 include/qemu-common.h |   11 +++++++
 hw/scsi/virtio-scsi.c |   76 +++++++++++++++++++++++++++++++++++++++++++++++--
 util/hexdump.c        |   20 +++++++++++++
 trace-events          |   19 ++++++++++++
 4 files changed, 123 insertions(+), 3 deletions(-)