Implement (most) of the client side (i.e. VMM side) of the vfio-user
protocol:
https://github.com/nutanix/libvfio-user/blob/master/docs/vfio-user.rst
Items that are not implemented (because they are optimisations or unused
due to alternative solutions:
* VFIO_USER_DMA_READ/WRITE - this is a way for the server to read guest
memory if the guest memory is not shared by fd where the client
doesn't support it. However since we do support sharing the memory by
fd this is not required.
* VFIO_USER_GET_REGION_IO_FDS - an optimisation to bypass the VMM by
having KVM talk directly to the backend using ioregionfd
* VFIO_USER_DIRTY_PAGES - for the implementation of live migration
Signed-off-by: Rob Bradford <robert.bradford@intel.com>
