virtio-pci: Add Function Level Reset support

Using FLR becomes convenient in cases where resetting the bus is
impractical, for example, when debugging the behavior of individual
functions.

Signed-off-by: Julia Suvorova <jusual@redhat.com>
Message-Id: <20190820163005.1880-1-jusual@redhat.com>
Reviewed-by: Michael S. Tsirkin <mst@redhat.com>
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>