README, xen/Makefile: Branching for 4.5

Change version numbers to 4.5-unstable.

Signed-off-by: Ian Jackson <Ian.Jackson@eu.citrix.com>
QEMU_TAG, QEMU_UPSTREAM_REVISION: Branching

QEMU_UPSTREAM_REVISION set back to master, to track the tip.

QEMU_TAG set to the specific changeset as is customary.

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