agl-kvm: Fix AGL_GUEST_MACHINE definition

agl-kvm: Fix AGL_GUEST_MACHINE definition

The override use for AGL_GUEST_MACHINE definition does not seem to
work reproducibly, so switch back to having virtio-aarch64 as a
default value, and provide a MACHINE based include hook to
potentially override it for specific host machine targets.

Bug-AGL: SPEC-5306

Change-Id: I76da46b8d02bf9ac0efc917fd401e1be3ac98906
Signed-off-by: Scott Murray <scott.murray@konsulko.com>
Reviewed-on: https://gerrit.automotivelinux.org/gerrit/c/AGL/meta-agl-demo/+/30544
ci-image-build: Jenkins Job builder account
ci-image-boot-test: Jenkins Job builder account
Reviewed-by: Jan-Simon Moeller <jsmoeller@linuxfoundation.org>
Tested-by: Jenkins Job builder account