If 'base' is smaller than the overlay image being committed into it,
then the base image will be grown in commit_run via bdrv_truncate().

This tests to make sure that this works, and the bdrv_truncate() is
not blocked when it shouldn't be.
Signed-off-by: NJeff Cody <jcody@redhat.com>
Reviewed-by: NEric Blake <eblake@redhat.com>
Signed-off-by: NKevin Wolf <kwolf@redhat.com>