vmxnet3: Support reading IMR registers on bar0 Instead of asserting, return the actual IMR register value. This is aligned with what's returned on ESXi. Signed-off-by: Shmulik Ladkani <shmulik.ladkani@ravellosystems.com> Tested-by: Dana Rubin <dana.rubin@ravellosystems.com> Signed-off-by: Jason Wang <jasowang@redhat.com>