vhost: Make vhost a separate module

Currently, vhost-net and vhost-scsi are sharing the vhost core code.
However, vhost-scsi shares the code by including the vhost.c file
directly.

Making vhost a separate module makes it is easier to share code with
other vhost devices.

Signed-off-by: Asias He <asias@redhat.com>
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>
diff --git a/drivers/vhost/test.c b/drivers/vhost/test.c
index 1ee45bc..9b71a57 100644
--- a/drivers/vhost/test.c
+++ b/drivers/vhost/test.c
@@ -18,7 +18,7 @@
 #include <linux/slab.h>
 
 #include "test.h"
-#include "vhost.c"
+#include "vhost.h"
 
 /* Max number of bytes transferred before requeueing the job.
  * Using this limit prevents one virtqueue from starving others. */