public/io/netif.h: add gref mapping control messages

public/io/netif.h: add gref mapping control messages

Adds 3 messages to allow guest to let backend keep grants mapped,
such that 1) guests allowing fast recycling of pages can avoid doing
grant ops for those cases, or otherwise 2) preferring copies over
grants and 3) always using a fixed set of pages for network I/O.

The three control ring messages added are:
 - Add grefs to be mapped by backend
 - Remove grefs mappings (If they are not in use)
 - Get maximum amount of grefs kept mapped.

Acked-by: Wei Liu <wei.liu2@citrix.com>
Reviewed-by: Paul Durrant <paul.durrant@citrix.com>
Reviewed-by: Konrad Rzeszutek Wilk <konrad.wilk@oracle.com>
Signed-off-by: Joao Martins <joao.m.martins@oracle.com>