qemu-option: Pair g_malloc() with g_free(), not free()

Spotted by Coverity with preview checker ALLOC_FREE_MISMATCH enabled
and my "coverity: Model g_free() isn't necessarily free()" model patch
applied.

Signed-off-by: Markus Armbruster <armbru@redhat.com>
Reviewed-by: Gonglei <arei.gonglei@huawei.com>
Reviewed-by: Eric Blake <eblake@redhat.com>
Signed-off-by: Michael Tokarev <mjt@tls.msk.ru>

util/qemu-option.c

@@ -230,7 +230,7 @@ bool has_help_option(const char *param)
     }
 
 out:
-    free(buf);
+    g_free(buf);
     return result;
 }
 
@@ -255,7 +255,7 @@ bool is_valid_option_list(const char *param)
     }
 
 out:
-    free(buf);
+    g_free(buf);
     return result;
 }