libxl: remove LIBXL_ATTR_UNUSED
libxl: remove LIBXL_ATTR_UNUSED

Unused as of:
  commit 446d09149802677546449fa2dd253f3ebce377ac
    libxl: pass driver config to libxlMakeDomBuildInfo

All other usage of LIBXL_HAVE_DEVICE_CHANNEL was removed by:
  commit e58004d70aceb560fba64803e566b8be3ef93940
    Xen: Remove unneeded LIBXL_HAVE_* ifdefs

Signed-off-by: Ján Tomko <4cab11cfb98d3c937327354a78eb07dbb6ee2bc6@redhat.com>
Reviewed-by: Michal Privoznik <83d82aaba2eed257f4814b0c239c260c4caaadf0@redhat.com>

olafhering/libvirt,crobinso/libvirt,crobinso/libvirt,zippy2/libvirt,olafhering/libvirt,zippy2/libvirt,jfehlig/libvirt,libvirt/libvirt,libvirt/libvirt,crobinso/libvirt,crobinso/libvirt,olafhering/libvirt,libvirt/libvirt,zippy2/libvirt,zippy2/libvirt,jfehlig/libvirt,libvirt/libvirt,olafhering/libvirt,jfehlig/libvirt,jfehlig/libvirt