coverity: update model for latest tools

Coverity is now rejecting incomplete types in the modeling file.
Just use a random number (in the neighborhood of the actual one)
for the size of a GIOChannel.

Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>

scripts/coverity-scan/model.c

@@ -356,7 +356,8 @@ int g_poll (GPollFD *fds, unsigned nfds, int timeout)
 typedef struct _GIOChannel GIOChannel;
 GIOChannel *g_io_channel_unix_new(int fd)
 {
-    GIOChannel *c = g_malloc0(sizeof(GIOChannel));
+    /* cannot use incomplete type, the actual struct is roughly this size.  */
+    GIOChannel *c = g_malloc0(20 * sizeof(void *));
     __coverity_escape__(fd);
     return c;
 }