qemu: Fix improper error message for disk detaching

s/virDomainDeviceTypeToString/virDomainDiskDeviceTypeToString/

Report by Xu He Jie <xuhj@linux.vnet.ibm.

src/qemu/qemu_driver.c

@@ -5212,8 +5212,8 @@ qemuDomainDetachDeviceDiskLive(struct qemud_driver *driver,
         break;
     default:
         qemuReportError(VIR_ERR_CONFIG_UNSUPPORTED,
-                        _("device type '%s' cannot be detached"),
-                        virDomainDeviceTypeToString(dev->type));
+                        _("disk device type '%s' cannot be detached"),
+                        virDomainDiskDeviceTypeToString(disk->type));
         break;
     }
     return ret;