kvm tools: add support for ARMv7 processors

This patch adds initial support for ARMv7 processors (more specifically,
Cortex-A15) to kvmtool.

Everything is driven by FDT, including dynamic generation of virtio nodes
for MMIO devices (PCI is not used due to lack of a suitable host-bridge).

The virtual timers and virtual interrupt controller (VGIC) are provided
by the kernel and require very little in terms of userspace code.

Tested-by: Marc Zyngier <marc.zyngier@arm.com>
Signed-off-by: Will Deacon <will.deacon@arm.com>
Signed-off-by: Pekka Enberg <penberg@kernel.org>