qemu: Update qemuDomainFindSCSIControllerModel return

qemu: Update qemuDomainFindSCSIControllerModel return

Now that the controller model is updated during post parse callback,
this code no longer needs to fetch the model based on the capabilities
and can just return the model directly if the controller is found.

Removal of @qemuCaps cascades through various callers which are now
updated to not pass the capabilities.