Support VMDK and VDI format

Add osv.vmdk and osv.vdi targets on Makefile.

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