Config.mk: update QEMU_TAG and QEMU_UPSTREAM_REVISION

Update these to the Xen 4.4.0 tags.  These refer to the same commits
as the previously-referenced tags.

Signed-off-by: Ian Jackson <Ian.Jackson@eu.citrix.com>
4.4 release: Update README

Update paragraph bragging about the features.  Also, remove the ".0"
from the end of the banner, so we don't have to keep updating it every
point release.

Signed-off-by: George Dunlap <george.dunlap@eu.citrix.com>