tap: add vhost/vhostfd options

This adds vhost binary option to tap, to enable vhost net accelerator.
Default is off for now, we'll be able to make default on long term
when we know it's stable.

vhostfd option can be used by management, to pass in the fd. Assigning
vhostfd implies vhost=on.

Signed-off-by: Michael S. Tsirkin <mst@redhat.com>
Signed-off-by: Anthony Liguori <aliguori@us.ibm.com>

net.c

@@ -976,6 +976,14 @@ static const struct {
                 .name = "vnet_hdr",
                 .type = QEMU_OPT_BOOL,
                 .help = "enable the IFF_VNET_HDR flag on the tap interface"
+            }, {
+                .name = "vhost",
+                .type = QEMU_OPT_BOOL,
+                .help = "enable vhost-net network accelerator",
+            }, {
+                .name = "vhostfd",
+                .type = QEMU_OPT_STRING,
+                .help = "file descriptor of an already opened vhost net device",
             },
 #endif /* _WIN32 */
             { /* end of list */ }