kvm tools: add generic device registration mechanism

PCI devices are currently registered into the pci_devices array via the
pci__register function, which can then be indexed later by architecture
code to construct device tree nodes. For MMIO devices, there is no such
utility.

Rather than invent a similar mechanism for MMIO, this patch creates a
global device registration mechanism, which allows the device type to be
specified when registered or indexing a device. Current users of the pci
registration code are migrated to the new infrastructure and virtio MMIO
devices are registered at init time.

As part of the device registration, allocation of the device number is
moved out of irq__register_device and performed when adding the device
header to the relevant bus tree, allowing us to maintain separate device
numberspaces for each bus.

Signed-off-by: Will Deacon <will.deacon@arm.com>
Signed-off-by: Pekka Enberg <penberg@kernel.org>