refactor: add top-level device manager

PCIe spec mandates that software can access the configuration space of
PCIe devices both via MMIO and Port IO accesses. As a result, PCIe
devices will need to register to both buses (on x86).

Change the organization of devices, so that MMIO and PIO device managers
do not own the buses. Instead, introduce a DeviceManager object which
holds the buses, the resource allocator and includes also all types of
device managers (at the moment MMIO, PIO and ACPI).

Signed-off-by: Babis Chalios <bchalios@amazon.es>

Author: bchalios