remove ruby-libvirt dependency

Signed-off-by: Hiroshi Miura <miurahr@linux.com>
Merge pull request #110 from jordant/master

Support for cpu_mode configuration while using the nested option.
Merge pull request #88 from miurahr/reomve_ruby_libvirt_dependency

remove ruby-libvirt dependency
Depend on ruby-libvirt gem since fog does not pull it in
Merge pull request #114 from sciurus/require_ruby_libvirt

Depend on ruby-libvirt gem since fog does not pull it in
Merge remote-tracking branch 'upstream/master'