vmxnet3: add support for capability registers

This patch enhances vmxnet3 to suuport capability registers which
allows it to enable features selectively. The DCR register tracks
the capabilities vmxnet3 device supports. The PTCR register states
the capabilities that the passthrough device supports.

With the help of these registers, vmxnet3 can enable only those
features which the passthrough device supoprts. This allows
smooth trasition to Uniform-Passthrough (UPT) mode if the virtual
nic requests it. If PTCR register returns nothing or error it means
UPT is not being requested and vnic will continue in emulation mode.

Signed-off-by: Ronak Doshi <doshir@vmware.com>
Acked-by: Guolin Yang <gyang@vmware.com>
Signed-off-by: Paolo Abeni <pabeni@redhat.com>