eal: query multi-vector interrupt capability

eal: query multi-vector interrupt capability

VFIO allows multiple MSI-X vector, others doesn't, but maybe will
allow it in the future.
Device drivers need to be aware of the capability.
It's better to avoid condition check on interrupt type (VFIO) everywhere,
instead a capability api is more flexible for the condition change.

Signed-off-by: Cunming Liang <cunming.liang@intel.com>
Acked-by: David Marchand <david.marchand@6wind.com>