my $p = XML::XPath->new (xml => $xml);
my @disks = $p->findnodes ('//devices/disk/source/@dev');
push (@disks, $p->findnodes ('//devices/disk/source/@file'));
my $p = XML::XPath->new (xml => $xml);
my @disks = $p->findnodes ('//devices/disk/source/@dev');
push (@disks, $p->findnodes ('//devices/disk/source/@file'));