block/null: Implement bdrv_refresh_filename()

block/null: Implement bdrv_refresh_filename()

The null block driver ignores any filename used for creating its BDSs,
which allows creating such BDSs even without any filename at all. In
that case, we currently construct a JSON filename when queried instead
of a plain "null-co://" or "null-aio://". This patch implements
bdrv_refresh_filename() to remedy this behavior.

Signed-off-by: Max Reitz <mreitz@redhat.com>
Message-id: 20160610185750.30956-4-mreitz@redhat.com
[mreitz@redhat.com: Added commit message]
Signed-off-by: Max Reitz <mreitz@redhat.com>