xsm: expose context of event channel peers

xsm: expose context of event channel peers

This hypercall allows a domain to identify the security context of a
domain that it is communicating with using the interdomain event
channel that it is using for the communication. This can be used to
augment Xen's security permissions with intra-domain security checks.

Signed-off-by: Daniel De Graaf <dgdegra@tycho.nsa.gov>
Committed-by: Keir Fraser <keir@xen.org>