Rather than repeat multiple steps in order to find the SCSI
controller model, let's combine them into one helper that will
return either the model from the definition or the default
model based on the capabilities.

This patch adds an extra check/error that the controller
that's being found actually exists. This just clarifies that
the error was because the controller doesn't exist rather
than the more generic error that we were unable to determine
the model from qemuDomainSetSCSIControllerModel when a -1
was passed in and the capabilities were unable to find one.