grant_table: implement grant_table_warn_active_grants()

grant_table: implement grant_table_warn_active_grants()

Log first 10 active grants for a domain. This function is going to be used
for soft reset, active grants on this path usually mean misbehaving backends
refusing to release their mappings on shutdown. We need that in addition to
the already existent 'g' keyhandler as such misbehaving backends can cause a
domain to crash right after the soft reset operation and 'g' option won't be
available in this case.

Signed-off-by: Vitaly Kuznetsov <vkuznets@redhat.com>
Reviewed-by: Konrad Rzeszutek Wilk <konrad.wilk@oracle.com>
Acked-by: Jan Beulich <jbeulich@suse.com>