VIR_FREE: Replace internals by g_clear_pointer

Our implementation masks GCC warnings of uninitialized use of the passed
argument. After changing this I got a load of following warnings:

src/conf/virnetworkportdef.c: In function 'virNetworkPortDefSaveStatus':
/usr/include/glib-2.0/glib/gmem.h:136:8: error: 'path' may be used uninitialized in this function [-Werror=maybe-uninitialized]
136 | if (_p) \
| ^
src/conf/virnetworkportdef.c:447:11: note: 'path' was declared here
447 | char *path;
| ^~~~

For the curious, g_clear_pointer is still safe for arguments with
side-effect. Here's the pre-processed output of trying to do a
VIR_FREE(*(test2++)):

do {
typedef char _GStaticAssertCompileTimeAssertion_1[(sizeof *(&(*(test2++))) == sizeof (gpointer)) ? 1 : -1] __attribute__((__unused__));
__typeof__((&(*(test2++)))) _pp = (&(*(test2++)));
__typeof__(*(&(*(test2++)))) _ptr = *_pp;

*_pp = ((void *)0);
if (_ptr)
(g_free) (_ptr);
} while (0) ;

Signed-off-by: Peter Krempa <pkrempa@redhat.com>
Reviewed-by: Ján Tomko <jtomko@redhat.com>