virtio_blk: Fix issuing T_GET_ID before DRIVER_OK status

DRIVER_OK status is set after device_attach() succeeds. For now postpone
disk_create to attach_completed() method.

MFC after: 2 weeks
Sponsored by: The FreeBSD Foundation
Reviewed by: grehan
Approved by: lwhsu (mentor)
Differential Revision: https://reviews.freebsd.org/D30049