kvm tools: Add virtio-mmio support

This patch is based on Sasha's 'kvm tools: Add support for virtio-mmio'
patch. ioeventfds support is added which was missing in the previous one.
VQ size/align is still not supported.

It adds support for the new virtio-mmio transport layer added in
3.2-rc1. The purpose of this new layer is to allow virtio to work on
systems which don't necessarily support PCI, such as embedded systems.

To apply the patch on top of the KVM tools tree, you must first pull
Linus' tree on top. Also, CONFIG_VIRTIO_MMIO=y should be set in the
guest kernel.

To easily test it it's recommended to apply Pawel Moll's patch named
'virtio-mmio: Devices parameter parsing' on top, and define the
virtio-mmio device using kernel command line.

LKVM will print a message to help user to figure out how to add kernel
command line to support virtio-mmio.

To instantiate guest virtio-mmio devices using kernel command line (or
module) parameter, e.g.

   virtio_mmio.devices=0x200@0xd2000000:5,0x200@0xd2000200:6

Cc: Pawel Moll <pawel.moll@arm.com>
Cc: Peter Maydell <peter.maydell@linaro.org>
Cc: Rusty Russell <rusty@rustcorp.com.au>
Cc: virtualization@lists.linux-foundation.org
Signed-off-by: Sasha Levin <levinsasha928@gmail.com>
Signed-off-by: Asias He <asias.hejun@gmail.com>
Signed-off-by: Pekka Enberg <penberg@kernel.org>