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>

doc/board/xen/xenguest_arm64.rst

@@ -23,6 +23,7 @@ previously done by NXP [4]:
 - PV block device frontend driver with XenStore based device enumeration and
   UCLASS_PVBLOCK class;
 - PV serial console device frontend driver;
+- Virtio block device support;
 - Xen hypervisor support with minimal set of the essential headers adapted from
   the Linux kernel;
 - Xen grant table support;
@@ -34,6 +35,7 @@ previously done by NXP [4]:
   define any start addresses at compile time which is up to Xen to choose at
   run-time;
 - new defconfig introduced: xenguest_arm64_defconfig.
+- new defconfig introduced: xenguest_arm64_virtio_defconfig.
 
 
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