xen-platform: Cleanup network infrastructure when emulated NICs are unplugged

When the guest unplugs the emulated NICs, cleanup the peer for each NIC
as it is not needed anymore. Most importantly, this allows the tap
interfaces which QEMU holds open to be closed and removed.

Signed-off-by: Ross Lagerwall <ross.lagerwall@citrix.com>
Acked-by: Anthony PERARD <anthony.perard@citrix.com>
Signed-off-by: Stefano Stabellini <sstabellini@kernel.org>