Migration code activates all block driver nodes on the destination when
the migration completes. It does so by calling
bdrv_invalidate_cache_all() and blk_resume_after_migration(). There is
one code path for precopy and one for postcopy migration, resulting in
four function calls, which used to have three different failure modes.

This patch unifies the behaviour so that failure to activate all block
nodes is non-fatal, but the error message is logged and the VM isn't
automatically started. 'cont' will retry activating the block nodes.

Signed-off-by: Kevin Wolf <kwolf@redhat.com>
Reviewed-by: Eric Blake <eblake@redhat.com>