Fix building agl-kvm-demo-platform for h3ulcb-kf

Fix building agl-kvm-demo-platform for h3ulcb-kf

Changes:
- Add masking the meta-rcar-gen3-adas layer to the KVM guest
  multiconfig to avoid picking up the dangling bbappends in
  meta-agl-bsp when building the guests.
- Remove the MACHINE_SUFFIX logic from the guest image copying in
  the agl-kvm-demo-platform recipe; it is a leftover from the
  original logic used in the IC EG container host images, and is
  unneeded here since the host and guests will be different
  MACHINEs (and the guest realistically cannot use the -kf
  machines).

Bug-AGL: SPEC-4760

Change-Id: If3237ca4d9fc86fc9d289a95bda24a4afcc7a991
Signed-off-by: Scott Murray <scott.murray@konsulko.com>