if image is a block device, the size check will fail the case because
QEMU fails to get correct size info from block device. So add a switch
to enable tester to turn off the snapshot size check.
Signed-off-by: Nlolyu <lolyu@redhat.com>

commit_snapshot_to_raw_backing: Add new parameters
qemu_disk_img: Add new parameters
commit_snapshot_to_raw_backing: New function for checking snapshot emptied after commitment
qemu_disk_img_commit: New function for checking snapshot emptied after commitment
Signed-off-by: NTingting Mao <timao@redhat.com>

1. rebase with different compat modes
2. rebase with various cache modes
Signed-off-by: NHaotong Chen <hachen@redhat.com>