iotests: Fix 291 across more file systems

iotests: Fix 291 across more file systems

Depending on the granularity of holes and amount of metadata consumed
by a file, the 'disk size:' number of 'qemu-img info' is not reliable.
Adjust our test to use a different set of filters to avoid spurious
failures.

Reported-by: Kevin Wolf <kwolf@redhat.com>
Fixes: cf2d1203dc
Signed-off-by: Eric Blake <eblake@redhat.com>
Message-Id: <20200608195629.3299649-1-eblake@redhat.com>
Acked-by: Kevin Wolf <kwolf@redhat.com>
[eblake: fix merge conflict]
Signed-off-by: Eric Blake <eblake@redhat.com>