Manage LVM locks

It's not possible de take 2 snapshot of the same LVM volume at the same time.
This is a problem when using a single file system (backed by a single LVM volume) to store
several VM as trying to run 2 backups at the same time will fail (at least, taking the snapshot for one will fail).

Add locking before taking a snapshot and wait up to 10 sec for the lock to be released. This should fix
concurrent backups

virt-backup

@@ -776,15 +776,33 @@ sub save_xml{
 sub create_snapshot{
     my ($blk,$suffix) = @_;
     my $ret = 0;
+    my $lock = $blk;
+    $lock =~ s/\//\-/g;
+    $lock = $opts{backupdir} . '/' . $lock . '.lock';
     my $cmd = "$opts{lvcreate} -s -n " . $blk . $suffix .
               " -L $opts{snapsize} $blk > /dev/null 2>&1 < /dev/null\n";
-    print "Running: $cmd" if $opts{debug};
-    if ( system("$cmd") == 0 ) {
-        $ret = 1;
-        open SNAPLIST, ">>$backupdir.meta/snapshots" or die "Error, couldn't open snapshot list file\n";
-        print SNAPLIST $blk.$suffix ."\n";
-        close SNAPLIST;
+    for ($cnt = 0; $cnt < 10; $cnt++ ){
+        print "Running: $cmd" if $opts{debug};
+        if (-e "$lock" . '.lock'){
+          print "Volume $blk is locked...\n" if $opts{debug};
+          sleep(1);
+        }
+        else{
+            open ( LOCK, ">$lock" );
+            print LOCK "";
+            close LOCK;
+            if ( system("$cmd") == 0 ) {
+                $ret = 1;
+                open SNAPLIST, ">>$backupdir.meta/snapshots" or die "Error, couldn't open snapshot list file\n";
+                print SNAPLIST $blk.$suffix ."\n";
+                close SNAPLIST;
+                # break the loop now
+                $cnt = 10;
+            }
+        }
     }
+    # In any case, failed or not, remove our lock
+    unlink <$lock>;
     return $ret;
 }