ethdev: add link state interrupt flag

ethdev: add link state interrupt flag

Only some devices support the link state interrupt configuration option.
Link state control does not work in virtual drivers
(virtio, vmxnet3, igbvf, and ixgbevf). Instead of having the application
try and guess whether it will work or not provide a driver flag that
can be checked instead.

Note: if device driver doesn't support link state control, what
would happen previously is that the code would never detect link
transitions. This prevents that.

Signed-off-by: Stephen Hemminger <stephen@networkplumber.org>
[Thomas: rename flag]
Acked-by: Thomas Monjalon <thomas.monjalon@6wind.com>