aarch64: first pci implementation

to make effective use of this implementation of PCI for AArch64
the QEMU patches to enable PCI in mach-virt by Rob Herring and
Alvise Rigo are necessary (and out of tree for now).

Signed-off-by: Claudio Fontana <claudio.fontana@huawei.com>
aarch64: enable virtio and virtio-blk

these patches allow early enablement of virtio and virtio-blk
for AArch64 with a fair amount of AARCH64_PORT_STUB defines.

The main issue is the "interrupt module", as loosely defined
by core/interrupt.cc and include/osv/interrupt.hh, which is
actually x64-specific.

Signed-off-by: Claudio Fontana <claudio.fontana@huawei.com>