print meaningful error message in case of --disable-vhost-net

When qemu gets compiled without support of vhost-net, any attempt
to use it fails with a very clear error message:

 qemu-system-x86_64: -netdev ...,vhost=on: vhost-net requested but could not be initialized

there's absolutely no reason given _why_ it coult not be
initialized, and even strace'ing the process in question
does not reveal any errors.  So print a message telling
what's going on.

Signed-off-by: Michael Tokarev <mjt@tls.msk.ru>
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>

1 files changed, 2 insertions, 0 deletions

diff --git a/hw/vhost_net.c b/hw/vhost_net.c
index e8c9e940d9..b6dc592db0 100644
--- a/hw/vhost_net.c
+++ b/hw/vhost_net.c
@@ -15,6 +15,7 @@
 
 #include "virtio-net.h"
 #include "vhost_net.h"
+#include "qemu-error.h"
 
 #include "config.h"
 
@@ -203,6 +204,7 @@ void vhost_net_cleanup(struct vhost_net *net)
 struct vhost_net *vhost_net_init(VLANClientState *backend, int devfd,
                                  bool force)
 {
+    error_report("vhost-net support is not compiled in");
     return NULL;
 }