conf: Report error when default TPM version is provided

conf: Report error when default TPM version is provided

When "default" version of 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 qemuDomainDefTPMsPostParse()).

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