kvm_x86_64: update INSTALL instructions and VM creation

Reviewed By: tbd

Updated the instructions for creating a new ONIE virtual machine.

Include new instructions for booting and embedding ONIE using the
recovery ISO image.  This method is much easier than the iPXE install
method.

Also updated the mk-vm.sh script.  Three boot modes are supported:

- boot from hard disk
- boot from CDROM (recovery ISO)
- boot from network (iPXE)