ehci: fix segfault when hot-unplugging ehci controller

ehci: fix segfault when hot-unplugging ehci controller

When hot-unplugging the usb controllers (ehci/uhci),
we have to clean all resouce of these devices,
involved registered reset handler. Otherwise, it
may cause NULL pointer access and/or segmentation fault
if we reboot the guest os after hot-unplugging.

Let's hook up reset via DeviceClass->reset() and drop
the qemu_register_reset() call. Then Qemu will register
and unregister the reset handler automatically.

Cc: qemu-stable <qemu-stable@nongnu.org>
Reported-by: Lidonglin <lidonglin@huawei.com>
Signed-off-by: Gonglei <arei.gonglei@huawei.com>
Signed-off-by: Gerd Hoffmann <kraxel@redhat.com>