(FACT-706) Fixing xenstore-read errors in rackspace.rb

If a vm has the xen-tools installed, but is not running from
rackspace, the following error would crop up at every interactive
run:

/usr/bin/xenstore-read:
couldn't read path vm-data/provider_data/provider

Redirecting stderr to /dev/null (similar to domain.rb, virtual.rb,
etc) alleviates this while still returning relevant results when
applicable.