conf: Report an error when default TPM model is provided

conf: Report an error when default TPM model is provided

When "default" model of a TPM was provided, our parses accepts it
happily even though the value is forbidden by our RNG and not
documented as accepted value. This is because of < 0 vs <= 0
comparison of virDomainTPMModelTypeFromString() retval.

Make the parser error out explicitly in this case. Users can
always chose to not specify the attribute in which case we pick a
sane default (in qemuDomainTPMDefPostParse()).

Signed-off-by: Michal Privoznik <mprivozn@redhat.com>
Reviewed-by: Peter Krempa <pkrempa@redhat.com>