When using an enum in a struct field, the compiler is free to decide to
make it an unsigned type if it desires. This in turn leads to bugs when
code does

    if ((def->foo = virDomainFooTypeFromString(str)) < 0)
       ...

because 'def->foo' can't technically have an unsigned value from the
compiler's POV. While it is possible to add (int) casts in the code
example above, this is not desirable because it is easy to miss out
such casts. eg the code fixed here caused an error with clang builds

../../src/conf/domain_conf.c:12838:73: error: comparison of unsigned enum expression < 0 is always false [-Werror,-Wtautological-compare]
        if ((def->version = virDomainTPMVersionTypeFromString(version)) < 0) {
            ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ ^ ~
Signed-off-by: NDaniel P. Berrangé <berrange@redhat.com>