KVM Provider

- removed check for available gems. Requiring fog is sufficient to get meaningful error messages if either the fog or libvirt gems are not present
- set libvirt_uri and libvirt_ip_command parameters for the Fog libvirt provider if they are not already set in the fog config file
- use the fog libvirt provider instead of a libvirt directly to make use of the configuration settings in .fog
- the availability of at least one storage pool and network is now checked through fog methods
- the libvirt version is now checked through the fog method
- the connection to libvirt is not closed as it gets automatically closed when the garbage collector deletes the variable
KVM Box

- removed requiring of libvirt and fog gems as this is now handled in the KVM provider class
- removed the options for creating the libvirt connection as these are now handled in the KVM provider class
KVM validate

- removed vbox tag and added kvm tag
KVM documentation

- extended the documentation with more details on prerequisites and usage of the kvm provider
Merge pull request #472 from ffeldhaus/kvm-provider

KVM provider: restructured to always use Fog.io to connect to libvirt