um: virt-pci: properly remove PCI device from bus

Triggering a bus rescan will not cause the PCI device to be removed. It
is required to explicitly stop and remove the device from the bus.

Fixes: 68f5d3f3 ("um: add PCI over virtio emulation driver")
Signed-off-by: Benjamin Berg <benjamin.berg@intel.com>
Signed-off-by: Richard Weinberger <richard@nod.at>