vmdk: Allow selecting SCSI adapter in image creation

vmdk: Allow selecting SCSI adapter in image creation

Introduce a new option "adapter_type" when converting to vmdk images.
It can be one of the following: ide (default), buslogic, lsilogic
or legacyESX (according to the vmdk spec from vmware).

In case of a non-ide adapter, heads is set to 255 instead of the 16.
The latter is used for "ide".

Also see LP#545089

Signed-off-by: Othmar Pasteka <pasteka@kabsi.at>
Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>