Update agl-virt feature

Update agl-virt feature

Changes:
- Add AGL_META_FILESYSTEMS hook to bblayers.conf.sample to match
  the other common meta-openembedded layer dependencies.
- Update agl-virt 50_bblayers.conf.inc to define the required
  AGL_META_* variables for the layers from meta-openembedded that
  it requires.

Bug-AGL: SPEC-3970

Signed-off-by: Scott Murray <scott.murray@konsulko.com>
Change-Id: Iae55fa082bd25cf43bde6152abab40c836cf6279
Reviewed-on: https://gerrit.automotivelinux.org/gerrit/c/AGL/meta-agl/+/26394
Reviewed-by: Jan-Simon Moeller <jsmoeller@linuxfoundation.org>
Tested-by: Jenkins Job builder account