refactor disk/storage checks for Disk API

Signed-off-by: Dominik Csapak <d.csapak@proxmox.com>

PVE/Storage.pm

@@ -1647,4 +1647,16 @@ sub get_bandwidth_limit {
     return $override;
 }
 
+# checks if the storage id is available and dies if not
+sub check_available {
+    my ($id) = @_;
+
+    my $cfg = config();
+    if (my $scfg = storage_config($cfg, $id, 1)) {
+	die "storage ID '$id' already defined\n";
+    }
+
+    return undef;
+}
+
 1;