vl: move global property, migrate init earlier

Currently drive_init_func() may call migrate_get_current() while the
migrate object is still not ready yet at that time. Move the migration
object init earlier, along with the global properties, right after
acceleration init.

This fixes a breakage for iotest 055, which caused an assertion failure.

Reported-by: Max Reitz <mreitz@redhat.com>
Reported-by: Philippe Mathieu-Daudé <f4bug@amsat.org>
Reviewed-by: Eduardo Habkost <ehabkost@redhat.com>
Tested-by: QingFeng Hao <haoqf@linux.vnet.ibm.com>
Fixes: 3df663 ("migration: move only_migratable to MigrationState")
Signed-off-by: Peter Xu <peterx@redhat.com>
Message-Id: <1499242883-2184-3-git-send-email-peterx@redhat.com>
Signed-off-by: Dr. David Alan Gilbert <dgilbert@redhat.com>