image-vm: Allow for xz compressed vmdk images

In order to save space, introduce a vmdk.xz IMAGE_FSTYPE.  We re-use
the existing xz compression logic in place as given how the images are
created we cannot use CONVERSION_CMD_xz directly.  We switch to using
bb.utils.contains in order to get an exact match on if vmdk.xz or vmdk
is in IMAGE_FSTYPES.

Change-Id: I9aa5fd1c8d80330536b9a38eaa154e3f706a3c2d
AGL-Issue: SPEC-514
Signed-off-by: Tom Rini <trini@konsulko.com>
Reviewed-on: https://gerrit.automotivelinux.org/gerrit/10277
Tested-by: Jenkins Job builder account <agl-jobbuilder@automotivelinux.org>
ci-image-build: Jenkins Job builder account <agl-jobbuilder@automotivelinux.org>
ci-image-boot-test: Jenkins Job builder account <agl-jobbuilder@automotivelinux.org>
Reviewed-by: Jan-Simon Moeller <jsmoeller@linuxfoundation.org>