(FACT-648) Cherry-pick lost Xen 4.0 command support back into master

This commit cherry-picks work done on the xendomains fact
back into master, which was lost during the facter-2 / master merge.

Original commit(a1dd388c4):
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.

Conflicts:
	lib/facter/util/xendomains.rb
	spec/unit/util/xendomains_spec.rb
Merge pull request #756 from whopper/xen_command-cherrypick

(Review) (FACT-648) Cherry-pick lost Xen 4.0 command support back into master