Add parameter to specify size of newly created disk

New options -s and --disksize to allow to specify the size of newly
created disks.
Add option to specify number of cpus

Add --cpu X option to pass how many cpus the virtual machine will use
Cleanup tabs, add -C as alias for --cpus

Thanks Adam for the review, here're the changes.
Enable CPU features

To support nested virtualization, you need to enable VMX.

saschpe, rhafer recommend:
  <cpu mode='custom' match='exact'>
     <model fallback='allow'>core2duo</model>
     <feature policy='require' name='vmx'/>
  </cpu>

I've added this as option to virt-install.
Merge pull request #14 from ajaeger/master

Add options for disk size and number of cpus