Allow CTL device specification in bhyve virtio-scsi.

There was a large refactoring done in CTL to allow multiple ioctl frontend
ports (and respective devices) to be created, particularly for bhyve.
Unfortunately, respective part of bhyve functionality got lost somehow from
the original virtio-scsi commit. This change allows wanted device path to
be specified in either of two ways:
-s 6,virtio-scsi,/dev/cam/ctl1.1
-s 6,virtio-scsi,dev=/dev/cam/ctl2.3
If neither is specified, the default /dev/cam/ctl device is used.

While there, remove per-queue CTL device opening, which makes no sense at
this point.

Reported by: wg
Reviewed by: araujo
MFC after: 3 days
Sponsored by: iXsystems, Inc.
Differential Revision: https://reviews.freebsd.org/D18504