board: xen: introduce virtio-blk support

Added new xenguest_arm64_virtio_defconfig which
enables support for virtio-blk using various types
of transport like virtio-pci, vrtio-mmio. Currently
supported: up to 2 PCI host bridges and 10 MMIO devices.
Note: DT parsing code was partly taken from pci-uclass.c
Limitation: All memory regions should be
below 4GB address space.

Signed-off-by: Andrii Chepurnyi <andrii_chepurnyi@epam.com>