kvm tools: Add abstract virtio transport layer

kvm tools: Add abstract virtio transport layer

Since virtio-mmio was introduced in 3.2, virtio-pci isn't the only transport
layer between the kernel and virtio devices.

This patch adds an abstract virtio-transport layer which allows to easily
use different transports while making it transparent to the device.

This is the first step in adding virtio-mmio support.

Cc: Pawel Moll <pawel.moll@arm.com>
Cc: Peter Maydell <peter.maydell@linaro.org>
Signed-off-by: Sasha Levin <levinsasha928@gmail.com>
Signed-off-by: Pekka Enberg <penberg@kernel.org>