qemu: Use same model when adding hostdev SCSI controller

qemu: Use same model when adding hostdev SCSI controller

When qemuDomainFindOrCreateSCSIDiskController adds a controller,
let's use the same model as a currently found controller under the
assumption that the reason to add the controller in hotplug is
because virDomainHostdevAssignAddress determined that there were
too many devices on the existing controller, but only assigned a
new controller index and did not add a new controller and we
desire to use the same controller model as any existing controller
and not take a chance that qemuDomainSetSCSIControllerModel would
use a default that may be incompatible.