qemu: snapshot: Correctly report qemu error on 'savevm'

Since 'savevm' was not converted to QMP libvirt has to parse for error
strings in the text monitor output. One of the unhandled errors is
produced when qemu treats a device as unmigratable.

As current qemu actually does support AHCI migration this bug is
applicable only to older versions of qemu.

Resolves: https://bugzilla.redhat.com/show_bug.cgi?id=1293899

src/qemu/qemu_monitor_text.c

@@ -2659,6 +2659,7 @@ qemuMonitorTextCreateSnapshot(qemuMonitorPtr mon,
 
     if (strstr(reply, "Error while creating snapshot") ||
         strstr(reply, "Could not open VM state file") ||
+        strstr(reply, "State blocked by non-migratable device") ||
         (strstr(reply, "Error") && strstr(reply, "while writing VM"))) {
         virReportError(VIR_ERR_OPERATION_FAILED,
                        _("Failed to take snapshot: %s"), reply);