build/image: warn if missing qemu-img for VDI/VMDK

build/image: warn if missing qemu-img for VDI/VMDK

Currently `qemu-img` is used to convert raw x86 images to VDI and VMDK
images, used for virtual machines.

Having `qemu-img` in tree requires us to maintain an ancient version of
`qemu-utils`, which recently required extra work to compile with newer
compiler version.

This commit prints a warning message in case `qemu-img` is missing.

As a next step the in-tree version of `qemu-img` can be removed.

Signed-off-by: Paul Spooren <mail@aparcar.org>