linux-agl-config: correct override syntax for virtualmachine

linux-agl-config: correct override syntax for virtualmachine

Commit dd6fc5dcaa0a027b7651bb365d5dd0f623498f8f as part of SPEC-4052 converted
all overrides to the new syntax, but missed the one used for "virtualmachine"

Bug-AGL: SPEC-4052
Change-Id: I1905c7e3b70b05c4ef06d8b4f240d0e144587fab
Signed-off-by: Denys Dmytriyenko <denys@konsulko.com>
Reviewed-on: https://gerrit.automotivelinux.org/gerrit/c/AGL/meta-agl/+/28204
Tested-by: Jenkins Job builder account
ci-image-build: Jenkins Job builder account
ci-image-boot-test: Jenkins Job builder account
Reviewed-by: Jan-Simon Moeller <jsmoeller@linuxfoundation.org>