modify resolve_disk_shortname() to always expect device name instead of index or name

  my $device = resolve_disk_shortname('disk1');
  my $device = resolve_disk_shortname('sda');
  my $device = resolve_disk_shortname('/dev/sda');

This patch also includes safeguard against accidental usage of 'disk0'.
('disk0' would be the last available device, which may lead to subtle bugs)
BUGFIX namespace issue when calling internal_error()