storage: Add check for valid FS types in checkPool callback

https://bugzilla.redhat.com/show_bug.cgi?id=1181087

The virStorageBackendFileSystemIsMounted is called from three source paths
checkPool, startPool, and stopPool. Both start and stop validate the FS
fields before calling *IsMounted; however the check path there is no call.
This could lead the code into returning a true in "isActive" if for some
reason the target path for the pool was mounted. The assumption being
that if it was mounted, then we believe we started/mounted it.

It's also of note that commit id '81165294' added an error message for
the start/mount path regarding that the target is already mounted so
fail the start. That check was adjusted by commit id '13fde7ce' to
only message if actually mounted.

At one time this led to the libvirtd restart autostart code to declare
that the pool was active even though the startPool would inhibit startup
and the stopPool would inhibit shutdown. The autostart path changed as
of commit id '2a31c5f0' as part of the keep storage pools started between
libvirtd restarts.

This patch adds the same check made prior to start/mount and stop/unmount
to ensure we have a valid configuration before attempting to see if the
target is already mounted to declare "isActive" or not. Finding an improper
configuration will now cause an error at checkPool, which should make it
so we can no longer be left in a situation where the pool was started and
we have no way to stop it.

src/storage/storage_backend_fs.c

@@ -537,6 +537,10 @@ virStorageBackendFileSystemCheck(virStoragePoolObjPtr pool,
     } else {
         int ret;
         *isActive = false;
+
+        if (virStorageBackendFileSystemIsValid(pool) < 0)
+            return -1;
+
         if ((ret = virStorageBackendFileSystemIsMounted(pool)) != 0) {
             if (ret < 0)
                 return -1;