vfio/pci: fix use of freed memory

hw/vfio/pci.c:308:29: warning: Use of memory after it is freed
        qemu_set_fd_handler(*pfd, NULL, NULL, vdev);
                            ^~~~

Reported-by: Clang Static Analyzer
Signed-off-by: Philippe Mathieu-Daudé <f4bug@amsat.org>
Reviewed-by: Paolo Bonzini <pbonzini@redhat.com>
Signed-off-by: Alex Williamson <alex.williamson@redhat.com>