conf: Use a temporary int variable to store GIC version

conf: Use a temporary int variable to store GIC version

Since no value in the virGICVersion enumeration is negative, a clever
enough compiler can report an error such as

  src/conf/domain_conf.c:15337:75: error: comparison of unsigned enum
  expression < 0 is always false [-Werror,-Wtautological-compare]
    if ((def->gic_version = virGICVersionTypeFromString(tmp)) < 0 ||
        ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ ^ ~

virGICVersionTypeFromString() can, however, return a negative value if
the input string is not part of the enumeration, so we definitely need
that check.

Work around the problem by storing the return value in a temporary int
variable.