ports: common utilities for building VMMs on NOVA

To ease the creation of custom virtual machine monitors on top of
NOVA, this patch moves generic utilities from vancouver resp. seoul to the
public include location 'ports/include/vmm'. As a nice side effect,
this change simplifies 'vancouver/main.cc'.

Issue #949
nova: support creation of vCPU in own PD

With the patch a VMM is not forced anymore to co-located the VMM and VM in same
PD.

Fixes #949
seoul: support to run VM outside VMM's PD

Issue #949

Related to issue #808 - one way to nearly double the maximum VM size for
VMs on 32bit Genode/Nova host if decreased performance is acceptable.
squash me to "ports: common utilities for building VMMs on NOVA"