hw/virtio: fix error enabling flags in Device Control register

When the virtio devices are PCI Express, make error-enabling flags
writable to respect the PCIe spec.

Signed-off-by: Marcel Apfelbaum <marcel@redhat.com>
Reviewed-by: Michael S. Tsirkin <mst@redhat.com>
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>