vmd(8): guard against bad virtio drivers

vmd(8): guard against bad virtio drivers

Add protections against guests with bad virtio-{blk,net,scsi}
drivers, specifically avoiding invalid descriptor chains and
invalid vionet packet sizes. This helps prevent possible lockup
of the host vm process due to a spinning device event loop thread.

Also fix an unneeded cast in the vioblk handling in case of invalid
buffer lengths.

OK mlarkin@