allow preexisting disk volumes via new option
Merge branches 'preexisting-disks', 'fix/disk-path' and 'extra-storage-cache-mode' into disks

Conflicts:
	README.md
	lib/vagrant-libvirt/config.rb
Merge branches 'allow-newer-ruby-libvirt', 'remove-default-provider', 'ssh-forward-logging' and 'disks' into working