virtio_scsi: use max target ID plus one as the initiator ID

This bus does not really have a concept of the initiator ID, so use
a guaranteed dummy one that won't conflict with any real target.

This change fixes a problem with virtio_scsi on GCE where disks get
sequential target IDs starting from one. If there are seven or more
disks, then a disk with the target ID of seven would not be discovered
by FreeBSD as that ID was reserved as the initiator ID -- see
scsi_scan_bus().

Discussed with: bryanv
MFC after: 2 weeks
Sponsored by: Panzura