High: Xen: Replace use of xm with xl (bnc#882548)

The xm command line tool has been deprecated in favor of the xl
toolstack.
For more information, see:
  - http://wiki.xen.org/wiki/MigrationGuideToXen4.1
  - http://wiki.xen.org/wiki/XL#Upgrading_from_xend

Use xl if available, else fall back on using xm.
Dev: Xen: Clean up trailing whitespace

Clean up trailing whitespace, and don't use ! as
heredoc delimiter as some editors don't handle it properly.