Check if qemu-bridge-helper exists and is executable

Otherwise the error is just

    error: Failed to create domain from test1.xml
    error: failed to retrieve file descriptor for interface: Transport endpoint is not connected

since we don't get a sensible error after the fork.

(cherry picked from commit 151ba02293)

src/qemu/qemu_command.c

@@ -296,6 +296,12 @@ static int qemuCreateInBridgePortWithHelper(virQEMUDriverConfigPtr cfg,
         return -1;
     }
 
+    if (!virFileIsExecutable(cfg->bridgeHelperName)) {
+        virReportSystemError(errno, _("'%s' is not a suitable bridge helper"),
+                             cfg->bridgeHelperName);
+        return -1;
+    }
+
     cmd = virCommandNew(cfg->bridgeHelperName);
     if (flags & VIR_NETDEV_TAP_CREATE_VNET_HDR)
         virCommandAddArgFormat(cmd, "--use-vnet");