kvm tools: arm: align guest memory buffer to maximum page size

If we're running a guest with a larger page size than the host,
interesting things start to happen when communicating via a virtio-mmio
device because the idea of buffer alignment between the guest and the
host will be off by the misalignment of the guest memory buffer allocated
by the host. This causes things like the index field of vring.used to
be accessed at different addresses on the guest and the host, leading
to deadlock.

Fix this problem by allocating guest memory aligned to the maximum
possible page size for the architecture (64K).

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