Introduce the virDomainOpenGraphics API

Introduce the virDomainOpenGraphics API

The virDomainOpenGraphics API allows a libvirt client to pass in
a file descriptor for an open socket pair, and get it connected
to the graphics display of the guest. This is limited to working
with local libvirt hypervisors connected over a UNIX domain
socket, since it will use UNIX FD passing

* include/libvirt/libvirt.h.in: Define virDomainOpenGraphics
* src/driver.h: Define driver for virDomainOpenGraphics
* src/libvirt_public.syms, src/libvirt.c: Entry point for
  virDomainOpenGraphics
* src/libvirt_internal.h: VIR_DRV_FEATURE_FD_PASSING