bhyve: change vq_getchain to return iovecs in both directions

bhyve: change vq_getchain to return iovecs in both directions

The old prototype requires callers to inspect flags of each descriptors
to get the starting position of host-writable iovecs.

vq_getchain() is changed to return a virtio request with the number of
host-readable iovecs and host-writable iovecs instead. Callers can avoid
boilerplate code of getting the start offset of host-writable iovecs.

Sponsored by:	The FreeBSD Foundation
MFC after:	3 weeks
Reviewed by:	afedorov
Approved by:	philip (mentor)
Differential Revision:	https://reviews.freebsd.org/D29433