xen_arm: Create virtio-mmio devices during initialization

In order to use virtio backends we need to allocate virtio-mmio
parameters (irq and base) and register corresponding buses.

Use the constants defined in public header arch-arm.h to be
aligned with the toolstack. So the number of current supported
virtio-mmio devices is 10.

For the interrupts triggering use already existing on Arm
device-model hypercall.

The toolstack should then insert the same amount of device nodes
into guest device-tree.

Signed-off-by: Oleksandr Tyshchenko <oleksandr_tyshchenko@epam.com>
Signed-off-by: Vikram Garhwal <vikram.garhwal@amd.com>
Reviewed-by: Stefano Stabellini <sstabellini@kernel.org>
Signed-off-by: Stefano Stabellini <stefano.stabellini@amd.com>