Xen 4.8.0-rc1 preparation

Xen 4.8.0-rc1 preparation

* Change QEMU_UPSTREAM_REVISION MINIOS_UPSTREAM_REVISION and
  QEMU_TRADITIONAL_REVISION to refer to the Xen 4.8.0-rc1 tags.

* Change README and xen/Makefile to refer to Xen 4.8.0-rc (note, the
  RC number is not included, so we do not have to update these again).

I reran autogen.sh as per the release checklist and this produced no
changes, as expected.  (Debian jessie i386.)

Signed-off-by: Ian Jackson <Ian.Jackson@eu.citrix.com>
Acked-by: Wei Liu <wei.liu2@citrix.com>