hw/xtensa: add virt machine

hw/xtensa: add virt machine

virt machine is a sim machine with generic PCI host controller.
Make common parts of sim machine initialization reusable.
Add PCI controller at 0xf0000000 with PIO space at its base address,
ECAM space at base address + 1M and MMIO space at base address + 64M.
Connect IRQ lines to consecutive CPU external IRQ pins starting from 0.
Instantiate network interfaces on virt machine.

Xtensa linux kernel configuration virt_defconfig can successfully boot
on this machine.

Signed-off-by: Max Filippov <jcmvbkbc@gmail.com>