ioreq-server: remove p2m entries when server is enabled

ioreq-server: remove p2m entries when server is enabled

For secondary servers, add a hvm op to enable/disable the server. The
server will not accept IO until it is enabled and the act of enabling
the server removes its pages from the guest p2m, thus preventing the guest
from directly mapping the pages and synthesizing ioreqs.

Signed-off-by: Paul Durrant <paul.durrant@citrix.com>
Acked-by: Ian Campbell <ian.campbell@citrix.com>
Acked-by: Jan Beulich <jbeulich@suse.com>