pci: add option for net failover

pci: add option for net failover

This patch adds a failover_pair_id property to PCIDev which is
used to link the primary device in a failover pair (the PCI dev) to
a standby (a virtio-net-pci) device.

It only supports ethernet devices. Also currently it only supports
PCIe devices. The requirement for PCIe is because it doesn't support
other hotplug controllers at the moment. The failover functionality can
be added to other hotplug controllers like ACPI, SHCP,... later on.

Signed-off-by: Jens Freimann <jfreimann@redhat.com>
Message-Id: <20191029114905.6856-3-jfreimann@redhat.com>
Reviewed-by: Michael S. Tsirkin <mst@redhat.com>
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>