hw/xen: only remove peers of PCI NICs on unplug

When the Xen guest asks to unplug *emulated* NICs, it's kind of unhelpful
also to unplug the peer of the *Xen* PV NIC.

Signed-off-by: David Woodhouse <dwmw@amazon.co.uk>
Reviewed-by: Paul Durrant <paul@xen.org>