iotests: Test qemu-img convert --salvage

iotests: Test qemu-img convert --salvage

This test converts a simple image to another, but blkdebug injects
block_status and read faults at some offsets.  The resulting image
should be the same as the input image, except that sectors that could
not be read have to be 0.

Signed-off-by: Max Reitz <mreitz@redhat.com>
Message-id: 20190507203508.18026-7-mreitz@redhat.com
Tested-by: Vladimir Sementsov-Ogievskiy <vsementsov@virtuozzo.com>
Reviewed-by: Vladimir Sementsov-Ogievskiy <vsementsov@virtuozzo.com>
[mreitz: Dropped superfluous printf from _filter_offsets, as suggested
         by Vladimir; disable test for VDI and IMGOPTSSYNTAX]
Signed-off-by: Max Reitz <mreitz@redhat.com>