Support xen 4.0 command invocation

As of Xen 4.0, querying for xen domains is done with `/usr/bin/xl`
while earlier versions used `/usr/bin/xm`. This commit checks over the
preferred command (xl) and then falls back to old command (xm) to detect
the correct command to run.
Merge branch 'pull-526'

This closes GH-526