Xen: Just specify 3.0 or above as a requirement, as versions above 3.1
have been tested and work fine with Ganeti. Also note that the Xen
dependency is optional, if one wants to work with KVM.

KVM: list as an alternative, specify that 72 (the one shipped with
lenny) is the bare minimum, but newer qemu-kvm is recommended.

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