make sure foreman doesnt blow up when running without libvirt / virt gems

fix for undefined constant virt error when running

When unattended is set to false, foreman is unable to run correctly.

This patch loads virt gem anyway (as bunder force it to be installed),
this is not ideal, but since we are migrating away from virt to use...

fixes #724 - Allow default VM specs to be tied to a hostgroup

fixes #843 - show more virtual guests information

added html view for guests
moved common methods into the virt plugin

fixes #835 - allow to delete virtual guests

added JSON output to guests
Added Guest model
Added confirmation before power cycle
Added Deletion option to guests