usb: fix serial generator

snprintf return value is *not* the number of chars written into the
buffer, but the number of chars needed.  So in case the buffer is too
small you can go alloc a bigger one and try again.  But that also means
you can't simply use the return value for the next snprintf call
without checking beforehand that things did actually fit.

Problem is that usb_desc_create_serial didn't perform that check, so a
loooong path string (can happen with deep pci-bridge nesting) results in
the third snprintf call smashing the stack.

Fix this by throwing out all the snpintf calls and use g_strdup_printf
instead.

https://bugzilla.redhat.com/show_bug.cgi?id=1381630

Reported-by: Thomas Huth <thuth@redhat.com>
Signed-off-by: Gerd Hoffmann <kraxel@redhat.com>
Reviewed-by: Thomas Huth <thuth@redhat.com>
Reviewed-by: Eric Blake <eblake@redhat.com>
Message-id: 1475659998-22045-1-git-send-email-kraxel@redhat.com
