Fix error detection in device change

According to qemu-kvm/qerror.c all messages start with a capital
"Device ", but the current code only scans for the lower case "device ".
This results in "virDomainUpdateDeviceFlags()" to not detect locked
CD-ROMs and reporting success even in the case of a failure:
	# virsh qemu-monitor-command "$VM" change\ drive-ide0-0-0\ \"/var/lib/libvirt/images/ucs_2.4-0-sec4-20110714145916-dvd-amd64.iso\"
	Device 'drive-ide0-0-0' is locked
	# virsh update-device "$VM" /dev/stdin <<<"<disk type='file' device='cdrom'><driver name='qemu' type='raw'/><source file='/var/lib/libvirt/images/ucs_2.4-0-sec4-20110714145916-dvd-amd64.iso'/><target dev='hda' bus='ide'/><readonly/><alias name='ide0-0-0'/><address type='drive' controller='0' bus='0' unit='0'/></disk>"
	Device updated successfully
Signed-off-by: NPhilipp Hahn <hahn@univention.de>

src/qemu/qemu_monitor_text.c

@@ -1081,7 +1081,7 @@ int qemuMonitorTextChangeMedia(qemuMonitorPtr mon,
     /* If the command failed qemu prints:
      * device not found, device is locked ...
      * No message is printed on success it seems */
-    if (strstr(reply, "device ")) {
+    if (c_strcasestr(reply, "device ")) {
         qemuReportError(VIR_ERR_OPERATION_FAILED,
                         _("could not change media on %s: %s"), devname, reply);
         goto cleanup;