vhost/scsi: Add ANY_LAYOUT iov -> sgl mapping prerequisites

This patch adds ANY_LAYOUT prerequisites logic for accepting a set of
protection + data payloads via iov_iter.  Also includes helpers for
calcuating SGLs + invoking vhost_scsi_map_to_sgl() with a known number
of iovecs.

Required by ANY_LAYOUT processing when struct iovec may be offset into
the first outgoing virtio-scsi request header.

Cc: Michael S. Tsirkin <mst@redhat.com>
Cc: Paolo Bonzini <pbonzini@redhat.com>
Cc: Al Viro <viro@zeniv.linux.org.uk>
Signed-off-by: Nicholas Bellinger <nab@linux-iscsi.org>