Only override the graphdriver to vfs if the priority is unset

This is an amend to containers#1460 That PR was
not addressing the case when the system wide config had the driver_priority
option configured and the user had no config file of their own. Then
`getRootlessStorageOpts` would be called and it would override the graph driver
to "vfs".

With this commit we only override the graph driver if driver priority is
empty. Otherwise we propagate the driver priority into the storage options, so
that the driver autodetection works as expected.

Signed-off-by: Dan Čermák <dcermak@suse.com>

types/options.go

@@ -299,8 +299,10 @@ func getRootlessStorageOpts(rootlessUID int, systemOpts StoreOptions) (StoreOpti
 			}
 		}
 	}
-	if opts.GraphDriverName == "" {
+	if opts.GraphDriverName == "" && len(systemOpts.GraphDriverPriority) == 0 {
 		opts.GraphDriverName = "vfs"
+	} else {
+		opts.GraphDriverPriority = systemOpts.GraphDriverPriority
 	}
 
 	if os.Getenv("STORAGE_OPTS") != "" {