hypervisors: switch to using HV_MIGRATION_PORT

This changes KVM to use HV_MIGRATION_PORT instead of KVM_MIGRATION_PORT
and enables passing the port for Xen migrations.

Since KVM_MIGRATION_PORT is not used anymore, we stop exporting it from
constants.py.

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