pcie/slot: fix hotplug event

When slot status register is cleared, PCIDevice::exp.hpev_notify
needs to be cleared.
Otherwise, PCIDevice::exp.hpev_notify is never set to false resulting
in no more hot plug event once it's raised.

Signed-off-by: Isaku Yamahata <yamahata@valinux.co.jp>
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>