don't monkey-patch #default_provider

This monkey-patching of Vagrant::Environment#default_provider breaks
with Vagrant 1.7.0, since the method's signature changed.  In any case
Vagrant should do the right thing without "help" from vagrant-libvirt.
Merge branches 'fix/disk-path', 'preexisting-disks', 'allow-newer-ruby-libvirt' and 'remove-default-provider' into working