bhyve: change vq_getchain to return read and write iovecs
The old prototype requires callers to inspect flags of each descriptors
to get the starting position of host-to-guest iovecs.
vq_getchain() is changed to return a virtio request with both the
guest-to-host iovecs and host-to-guest iovecs recorded instead.
Callers can obtain the host-to-guest iovecs directly.
Sponsored by: The FreeBSD Foundation
MFC after: 3 weeks