qemu: Move PCI command modelName check to controller def validate

qemu: Move PCI command modelName check to controller def validate

Move the various modelName == NAME_NONE from the command line
generation into domain controller validation.  Also rather than
have multiple cases with the same check, let's make the code
more generic, but also note that it was the modelName option
that caused the failure. We also have to be sure not to check
the PCI models that we don't care about.

For the remaining checks in command line building, we can use
the field name in the error message to be more specific about
what causes the failure.