vhost: split out vringh Kconfig

vringh is pulled in by caif and mic, but the other
vhost config does not need to be there.
In particular, it makes no sense to have vhost net/scsi/sock
under caif/mic.

Create a separate Kconfig file and put vringh bits there.

Signed-off-by: Michael S. Tsirkin <mst@redhat.com>