net/vmxnet3: support MSI-X interrupt

net/vmxnet3: support MSI-X interrupt

Add support for MSI-X interrupt vectors to the vmxnet3 driver.
This will allow more efficient deployments in cloud environments.

By default it will try to allocate 1 vector (0) for link
event and one MSI-X vector for each Rx queue.  To simplify
things, it will only be enabled if the number of Tx and Rx
queues are equal (so that Tx/Rx share the same vector).
If for any reason vmxnet3 cannot enable intr mode, it will
fall back to the LSC only mode.

Signed-off-by: Yong Wang <yongwang@vmware.com>
Signed-off-by: Jochen Behrens <jbehrens@vmware.com>