include/xen: import vendor extension to netif.h

include/xen: import vendor extension to netif.h

Describe in the protocol headers the extension we're making
with respect to staging grants. The extensions here described
are a middle ground with what is being discussed upstream
while keeping similar (yet different naming) structures
to be proposed upstream. The difference with upstream proposal
is that the staging grants occurs through a control ring;
here we do at xenbus features negotiation, which is more
maintainable while we keep this code out of tree.

Signed-off-by: Joao Martins <joao.m.martins@oracle.com>
Reviewed-by: Shannon Nelson <shannon.nelson@oracle.com>
Acked-by: Konrad Rzeszutek Wilk <konrad.wilk@oracle.com>
Orabug: 26107942