mpath: Update path in CheckPool function

https://bugzilla.redhat.com/show_bug.cgi?id=1230664

Per the devmapper docs, use "/dev/mapper" or "/dev/dm-n" in order to
determine if a device is under control of DM Multipath.

So add "/dev/mapper" to the virFileExists, leaving the "/dev/mpath"
as a "legacy" option since it appears for a while it was the preferred
mechanism, but is no longer maintained

src/storage/storage_backend_mpath.c

@@ -248,7 +248,8 @@ static int
 virStorageBackendMpathCheckPool(virStoragePoolObjPtr pool ATTRIBUTE_UNUSED,
                                 bool *isActive)
 {
-    *isActive = virFileExists("/dev/mpath");
+    *isActive = virFileExists("/dev/mapper") ||
+                virFileExists("/dev/mpath");
     return 0;
 }