coverity: Model GLib string allocation partially

Without a model, Coverity can't know that the result of g_strdup()
needs to be fed to g_free().

One way to get such a model is to scan GLib, build a derived model
file with cov-collect-models, and use that when scanning QEMU.
Unfortunately, the Coverity Scan service we use doesn't support that.

Thus, we're stuck with the other way: write a user model.  Doing that
for all of GLib is hardly practical.  I'm doing it for the "String
Utility Functions" we actually use that return dynamically allocated
strings.

In a local scan, this flags 20 additional RESOURCE_LEAKs.  The ones I
checked look genuine.

It also loses a NULL_RETURNS about ppce500_init() using
qemu_find_file() without error checking.  I don't understand why.

Signed-off-by: Markus Armbruster <armbru@redhat.com>
Acked-by: Paolo Bonzini <pbonzini@redhat.com>
