Support customize directive in Vagrantfile

- user now can call virsh to customize virtual machine
  ex.
  config.vm.provider :kvm do |kvm|
    kvm.customize ["-v"]
  end

Signed-off-by: Hiroshi Miura <miurahr@linux.com>
update README about customize parameter

Signed-off-by: Hiroshi Miura <miurahr@linux.com>
Merge branch 'customize' into next