There was a double free issue caused by virSysinfoRead on s390,
as the same manufacturer string instance was assigned to more
than one processor record.
Cleaned up other potential memory issues and restructured the sysinfo
parsing code by moving repeating patterns into a helper function.

The restructuring made it necessary to conditionally disable
-Wlogical-op for some older GCC versions, using pragma GCC diagnostic.
This is a GCC specific pragma, which is acceptable, since we're
using it to work around a GCC specific bug.

Finally, added a function virSysinfoSetup to configure the sysinfo
data source files/script during run time, to facilitate writing test
programs. This function is not published in sysinfo.h and only
there for testing.
Signed-off-by: NViktor Mihajlovski <mihajlov@linux.vnet.ibm.com>