Vagrantfile: Bump RAM to 2048, update comment

I was running out of RAM doing builds in the VM with just 512MB.
I think for single host machines, we can assume a lot of RAM.
If one is doing multi-node Vagrant that's a different thing.

Also update the comment at the top.

Closes: #313
Approved by: jlebon

Vagrantfile

@@ -1,16 +1,17 @@
 # vi: set ft=ruby :
 
-# In the future, this might do something more interesting
-# like contain code to sync git from the local dir into
-# the system and do builds/installs there.  But for
-# now at least this exists as a starting point, and
-# once `ostree admin unlock` exists in the next version,
-# things will be a bit simpler.
+# See `tests/vmcheck/README.md` for more information on this.
 
 Vagrant.configure(2) do |config|
     config.vm.box = "centos/atomic-host"
     config.vm.hostname = "centosah-dev"
 
+    config.vm.provider "libvirt" do |libvirt, override|
+      libvirt.cpus = 2
+      libvirt.memory = 2048
+      libvirt.driver = 'kvm'
+    end
+
     config.vm.provision "ansible" do |ansible|
       ansible.playbook = "tests/vmcheck/setup.yml"
       ansible.host_key_checking = false