Config.mk: update QEMU_TAG and QEMU_UPSTREAM_REVISION

Update these to the -rc6 tags.  These refer to the same commits as the
previously-referenced tags.

Signed-off-by: Ian Jackson <Ian.Jackson@eu.citrix.com>
README, xen/Makefile: Change in-tree version numbers

Change these to 4.4.0 in preparation for the release.  (We can still
make a change to the released codebase if something turns up between
now and the actual release.)

Signed-off-by: Ian Jackson <Ian.Jackson@eu.citrix.com>