bhyve virtio-scsi: Avoid out of bounds accesses to guest requests.

• Ignore I/O requests with insufficiently sized input or output buffers (those not containing compete request headers).

• Ignore control requests with improperly sized buffers.

• While here, explicitly zero the output header of an I/O request to avoid leaking malloc garbage from the host if the header is not fully populated.

PR: 264521
Reported by: Robert Morris <rtm@lcs.mit.edu>
Reviewed by: mav, emaste
MFC after: 1 week
Sponsored by: The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D36271

(cherry picked from commit bb31aee26bd13307d97c5d5bf2b10bf05bdc18fd)