build: Skip rebuild of usr.img on osv.vmdk and osv.vdi target

Support the following work flow

$ make image=netperf
$ make osv.vmdk

BTW, We should rename usr.img to osv.qcow2. But let's do it after this
release since a lot of scripts use the hardcoded usr.img naming.

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