qemu: catch failure of drive_add

Previously when QEMU failed "drive_add" due to an error opening
a file it would report

  "could not open disk image"

These days though, QEMU reports

  "Could not open '/tmp/virtd-test_e3hnhh5/disk1.qcow2': Permission denied"

which we were not detecting as an error condition.
Signed-off-by: NDaniel P. Berrange <berrange@redhat.com>

src/qemu/qemu_monitor_text.c

@@ -1959,6 +1959,16 @@ int qemuMonitorTextAddDrive(qemuMonitorPtr mon,
         goto cleanup;
     }
 
+    if (strstr(reply, "Could not open")) {
+        size_t len = strlen(reply);
+        if (reply[len - 1] == '\n')
+            reply[len - 1] = '\0';
+
+        virReportError(VIR_ERR_OPERATION_FAILED, "%s",
+                       reply);
+        goto cleanup;
+    }
+
     ret = 0;
 
  cleanup: