* qemud/remote.c, qemud/remote_protocol.x, src/driver.h,
          src/internal.h, src/libvirt.c, src/libvirt_sym.version,
          src/qemu_driver.c, src/remote_internal.c, src/test.c:
          Add a private interface so that libvirt.c coordination
          functions can detect the availability of features in
          underlying drivers, using the VIR_DRV_SUPPORTS_FEATURE
          macro.