block: Add bdrv_make_absolute_filename()

This is a general function for making a filename that is relative to a
certain BDS absolute.

It calls bdrv_get_full_backing_filename_from_filename() for now, but
that will be changed in a follow-up patch.

Signed-off-by: Max Reitz <mreitz@redhat.com>
Reviewed-by: Alberto Garcia <berto@igalia.com>
Message-id: 20190201192935.18394-13-mreitz@redhat.com
Signed-off-by: Max Reitz <mreitz@redhat.com>