build-sys: add --disable-vhost-user

Learn to compile out vhost-user (net, scsi & upcoming users). Keep it
enabled by default on non-win32, that is assumed to be POSIX. Fail if
trying to enable it on win32.

When trying to make a vhost-user netdev, it gives the following error:

-netdev vhost-user,id=foo,chardev=chr-test: Parameter 'type' expects a netdev backend type

And similar error with the HMP/QMP monitors.

While at it, rename CONFIG_VHOST_NET_TEST CONFIG_VHOST_USER_NET_TEST
since it's a vhost-user specific variable.

Signed-off-by: Marc-André Lureau <marcandre.lureau@redhat.com>
Reviewed-by: Cornelia Huck <cohuck@redhat.com>
Reviewed-by: Michael S. Tsirkin <mst@redhat.com>
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>