nbd: Use g_autofree in a few places

Thanks to our recent move to use glib's g_autofree, I can join the
bandwagon.  Getting rid of gotos is fun ;)

There are probably more places where we could register cleanup
functions and get rid of more gotos; this patch just focuses on the
labels that existed merely to call g_free.

Signed-off-by: Eric Blake <eblake@redhat.com>
Message-Id: <20190824172813.29720-2-eblake@redhat.com>
Reviewed-by: Daniel P. Berrangé <berrange@redhat.com>
Reviewed-by: Vladimir Sementsov-Ogievskiy <vsementsov@virtuozzo.com>