Document boot_order syntax for kvm

The gnt-instance manpage only contained the correct syntax for xen-pvm.
Specify what the kvm syntax is, and also warn about a problem with
virtio+netboot, for older kvm versions.

Signed-off-by: Guido Trotter <ultrotter@google.com>
Reviewed-by: Iustin Pop <iustin@google.com>