Set the libvirt driver to use accelerated kvm.

This wasn't necessary in previous versions of vagrant-libvirt because
this option specified the uri, and the domain type defaulted to kvm. In
newer releases, you have to specify this for performance, otherwise it
will default to qemu software only virtualization. This depends on:

vagrant-libvirt/vagrant-libvirt@67734f0

also available at:

purpleidea/vagrant-libvirt@67734f0

vagrant/gluster/Vagrantfile

@@ -575,7 +575,7 @@ Vagrant.configure(VAGRANTFILE_API_VERSION) do |config|
 	#	libvirt
 	#
 	config.vm.provider :libvirt do |libvirt|
-		libvirt.driver = 'qemu'
+		libvirt.driver = 'kvm'	# needed for kvm performance benefits !
 		# leave out to connect directly with qemu:///system
 		#libvirt.host = 'localhost'
 		libvirt.connect_via_ssh = false