image: Add vmdk.xz to the list of types that trigger image-vm inherit

image: Add vmdk.xz to the list of types that trigger image-vm inherit

Given how vmdk.xz is implemented currently it must also be added to the
list of image types that will cause image-vm to be inherited.

Change-Id: Ief017849c13c1d79c12411f39046389b1cc8a570
Signed-off-by: Tom Rini <trini@konsulko.com>
Reviewed-on: https://gerrit.automotivelinux.org/gerrit/10359
Tested-by: Jenkins Job builder account <agl-jobbuilder@automotivelinux.org>
ci-image-build: Jenkins Job builder account <agl-jobbuilder@automotivelinux.org>
Reviewed-by: Jan-Simon Moeller <jsmoeller@linuxfoundation.org>