Introduce HV_MIGRATION_PORT hypervisor parameter

This parameter will replace the direct use of KVM_MIGRATION_PORT and the
implicit use of the Xen migration port.

While it doesn't make sense to change this at instance level, we don't
have any other infrastructure for cluster-wide hypervisor parameters, so
we add it here (and document that it usually shouldn't be changed on a
per-instance basis).

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