The docs are ambiguous about the difference (or actually their
equality) between options '-virtfs' vs. '-fsdev'. So clarify that
'-virtfs' is actually just a convenience shortcut for its
generalized form '-fsdev' in conjunction with '-device virtio-9p-pci'.

And as we're at it, also be a bit more descriptive what 9pfs is
actually used for.

Signed-off-by: Christian Schoenebeck <qemu_oss@crudebyte.com>
Acked-by: Cornelia Huck <cohuck@redhat.com>
Message-Id: <208f1fceffce2feaf7c900b29e326b967dce7762.1585661532.git.qemu_oss@crudebyte.com>
Signed-off-by: Greg Kurz <groug@kaod.org>