In preparation of QAPI-fying VDI image creation, we have to create a
BlockdevCreateOptionsVdi type which is received by a (future)
vdi_co_create().

vdi_co_create_opts() now converts the QemuOpts object into such a
BlockdevCreateOptionsVdi object.  The protocol-layer file is still
created in vdi_co_do_create() (and BlockdevCreateOptionsVdi.file is set
to an empty string), but that will be addressed by a follow-up patch.

Note that cluster-size is not part of the QAPI schema because it is not
supported by default.
Signed-off-by: NMax Reitz <mreitz@redhat.com>
Signed-off-by: NKevin Wolf <kwolf@redhat.com>