net/virtio: disable LSC interrupt if MSIX not enabled

net/virtio: disable LSC interrupt if MSIX not enabled

The link state change interrupt can only be configured if the virtio device
supports MSIX.  Prior to this change the writing of the vector to the PCI
config space was causing it to overwrite the initial part of the MAC
address since the MSIX vector is not in the config space and is occupied by
the MAC address.

This has been reproduced in Virtual Box (v5.0.30.r112061) in Windows 7.

Fixes: 954ea11540b6 ("virtio: do not report link state feature unless available")
Cc: stable@dpdk.org
Signed-off-by: Matt Peters <matt.peters@windriver.com>
Signed-off-by: Allain Legacy <allain.legacy@windriver.com>