bhyve virtio-scsi: Tidy warning and debug prints.

Use a consistent prefix ("virtio-scsi: ") similar to the e1000 device
model.

Reviewed by: mav, emaste
MFC after: 1 week
Sponsored by: The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D36270

(cherry picked from commit 62806a7f31e3b4813c4e6b70534c41f91bfe4e12)