set vm name to avoid collisions

This avoids vm-name collisions in kvm identically to virtualbox..

Bit of a before-and-after:

````
% virsh list
 Id    Name                           State
----------------------------------------------------
 34    ubuntu-cloudimg-raring-vagrant-amd64 running
 36    vagrant-kvm_1372008185         running
 37    vagrant-kvm_1372008226         running
````

#34 was created before the patch, showing old style.  #36 and #37 were both created off of the same vagrantfile using the same raring64 box as #34.  Not tested much beyond 'vagrant up' and 'vagrant destroy' yet.

I also included pry as a dev gem because I find that useful, but as a separate commit...