iotests: Add test for relative backing file names

iotests: Add test for relative backing file names

Sometimes, qemu does not have a filename to work with, so it does not
know which directory to use for a backing file specified by a relative
filename. Add a test which tests that qemu exits with an appropriate
error message.

Additionally, add a test for qemu-img create with a backing filename
relative to the backed image's base directory while omitting the image
size.

Signed-off-by: Max Reitz <mreitz@redhat.com>
Reviewed-by: Eric Blake <eblake@redhat.com>
Reviewed-by: Fam Zheng <famz@redhat.com>
Signed-off-by: Kevin Wolf <kwolf@redhat.com>