migration: Add block-bitmap-mapping parameter

migration: Add block-bitmap-mapping parameter

This migration parameter allows mapping block node names and bitmap
names to aliases for the purpose of block dirty bitmap migration.

This way, management tools can use different node and bitmap names on
the source and destination and pass the mapping of how bitmaps are to be
transferred to qemu (on the source, the destination, or even both with
arbitrary aliases in the migration stream).

While touching this code, fix a bug where bitmap names longer than 255
bytes would fail an assertion in qemu_put_counted_string().

Suggested-by: Vladimir Sementsov-Ogievskiy <vsementsov@virtuozzo.com>
Signed-off-by: Max Reitz <mreitz@redhat.com>
Message-Id: <20200820150725.68687-2-mreitz@redhat.com>
Reviewed-by: Vladimir Sementsov-Ogievskiy <vsementsov@virtuozzo.com>
Signed-off-by: Eric Blake <eblake@redhat.com>