migration: fix bitmaps pre-blockdev migration with mirror job

migration: fix bitmaps pre-blockdev migration with mirror job

Important thing for bitmap migration is to select destination block
node to obtain the migrated bitmap.

Prepatch, on source we use bdrv_get_device_or_node_name() to identify
the node, and on target we do bdrv_lookup_bs.
bdrv_get_device_or_node_name() returns blk name only for direct
children of blk. So, bitmaps of direct children of blks are migrated by
blk name and others - by node name.

Old libvirt is unprepared to bitmap migration by node-name,
node-names are mostly auto-generated. So actually only migration by blk
name works for it.

Newer libvirt will use new interface (which will be added soon) to
specify node-mapping for bitmaps migration explicitly. Still, let's
improve the current behavior a bit.

Now, consider classic libvirt migrations assisted by mirror block job:
mirror block job inserts filter, so our source is not a direct child of
blk, and bitmaps are migrated by node-names. And this just doesn't work
with auto-generated node names.

Let's fix it by using blk-name even if some implicit filters are
inserted.

Note2: we, of course, can't skip filters and use blk name to migrate
bitmaps in filtered node by blk name for this blk if these filters have
named bitmaps which should be migrated.

Fixes: https://bugzilla.redhat.com/show_bug.cgi?id=1652424
Signed-off-by: Vladimir Sementsov-Ogievskiy <vsementsov@virtuozzo.com>
Message-Id: <20200521220648.3255-4-vsementsov@virtuozzo.com>
Reviewed-by: Eric Blake <eblake@redhat.com>
[eblake: comment typo fix, shorter subject line]
Signed-off-by: Eric Blake <eblake@redhat.com>