build: Support VMDK and other image formats

Current makefile cannot accept "make img_format=vmdk external all",
because QEMU does not support the format, but mkzfs.py and
upload_manifest.py requires to run OSv on QEMU.

With this patch, image conversion performs after mkzfs.py and
upload_manifest.py, prevents an error on QEMU execution.

Tested on QEMU/KVM and VMware, using these formats: raw / QCOW2 / VMDK.

Probably it can work on another formats such as VDI used by VirtualBox.

Signed-off-by: Takuya ASADA <syuu@cloudius-systems.com>
Signed-off-by: Pekka Enberg <penberg@cloudius-systems.com>