Fix a bug when build kvm box.

The error message is like:
==========================
		Transferring /tmp/.veewee_version20140523-4519-1mh5ulu to .veewee_version
		Error transfering file /tmp/.veewee_version20140523-4519-1mh5ulu failed, possible not enough permissions to write? SCP did not finish successfully (127):
		SCP did not finish successfully (127):
==========================
Turns out the guest machine doesn't have `scp` installed.