Rework virtio-aarch64 machine definition

Rework virtio-aarch64 machine definition

Start on potentially removing separate virtio-arch64 machine by
simplifying it and moving as much of its virtio kernel configuration
over to our generic kernel configuration scheme.  From some
experimentation, it is not obvious that genericarm64 or qemuarm64
are directly reusable as guest MACHINEs as is, so further
investigation is required on whether maintaining our own machine
and potentially custom kernel metadata cache is perhaps worthwhile.

Changes:
- Update virtio-aarch64 linux-yocto bbappend for new version in
  scarthgap, and drop its use of a custom kernel metadata cache for
  reusing qemuarm64's BSP as a starting point for now.
- Move the various virtio-*.cfg kernel configuration fragments over to
  meta-agl-core, using a new "agl-virtio-guest" AGL_FEATURES flag to
  control whether they are used.  The aim is to make it more obvious
  where any virtio kernel configuration comes from and hopefully make
  it more easily reusable.

Bug-AGL: SPEC-5137, SPEC-5201

Change-Id: I861cd1f80643aca632a5ab103eae2cc46de3922e
Signed-off-by: Scott Murray <scott.murray@konsulko.com>
Reviewed-on: https://gerrit.automotivelinux.org/gerrit/c/AGL/meta-agl/+/30119
ci-image-build: Jenkins Job builder account
Tested-by: Jenkins Job builder account
Reviewed-by: Jan-Simon Moeller <jsmoeller@linuxfoundation.org>
ci-image-boot-test: Jenkins Job builder account