Add a virt-host-validate command to sanity check HV config

Add a virt-host-validate command to sanity check HV config

To assist people in verifying that their host is operating in an
optimal manner, provide a 'virt-host-validate' command. For each
type of hypervisor, it will check any pre-requisites, or other
good recommendations and report what's working & what is not.

eg

  # virt-host-validate
  QEMU: Checking for device /dev/kvm                                         : FAIL (Check that the 'kvm-intel' or 'kvm-amd' modules are loaded & the BIOS has enabled virtualization)
  QEMU: Checking for device /dev/vhost                                       : WARN (Load the 'vhost_net' module to improve performance of virtio networking)
  QEMU: Checking for device /dev/net/tun                                     : PASS
   LXC: Checking for Linux >= 2.6.26                                         : PASS

This warns people if they have vmx/svm, but don't have /dev/kvm. It
also warns about missing /dev/vhost net.