qemu: sanity check vhost user FD before passing to QEMU

Ensure that the FD we're passing to QEMU is actually open, so we get a
sane error message upfront instead of telling QEMU to use a closed FD.

Reviewed-by: Michal Privoznik <mprivozn@redhat.com>
Signed-off-by: Daniel P. Berrangé <berrange@redhat.com>

src/qemu/qemu_command.c

@@ -4704,6 +4704,12 @@ qemuBuildVhostUserChardevStr(const char *alias,
 {
     char *chardev = NULL;
 
+    if (*fd == -1) {
+        virReportError(VIR_ERR_INTERNAL_ERROR, "%s",
+                       _("Attempt to pass closed vhostuser FD"));
+        return NULL;
+    }
+
     if (virAsprintf(&chardev, "socket,id=chr-vu-%s,fd=%d", alias, *fd) < 0)
         return NULL;