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