qemux86-64: root using root=PARTUUID

Virtualization machines expose the root device as differing devices
(e.g. /dev/sdaX versus /dev/hdaX). Update the syslinux.cfg to use
root=PARTUUID=foo as the root filesystem where foo is generated
by OE for us. This makes our image portable across VMs.

Bug-AGL: SPEC-514

Change-Id: I57447334ef76cd013213f3e80f6e71e77b58efa3
Signed-off-by: Matt Porter <mporter@konsulko.com>
Reviewed-on: https://gerrit.automotivelinux.org/gerrit/9031
Reviewed-by: Martin Kelly <mkelly@xevo.com>
Tested-by: Jenkins Job builder account <agl-jobbuilder@automotivelinux.org>
ci-image-build: Jenkins Job builder account <agl-jobbuilder@automotivelinux.org>
ci-image-boot-test: Jenkins Job builder account <agl-jobbuilder@automotivelinux.org>
Reviewed-by: Thomas Rini <tom.rini@gmail.com>
Reviewed-by: Jan-Simon Moeller <jsmoeller@linuxfoundation.org>