xsm/flask: add domain relabel support

xsm/flask: add domain relabel support

This adds the ability to change a domain's XSM label after creation.
The new label will be used for all future access checks; however,
existing event channels and memory mappings will remain valid even if
their creation would be denied by the new label.

With appropriate security policy and hooks in the domain builder, this
can be used to create domains that the domain builder does not have
access to after building. It can also be used to allow a domain to
drop privileges - for example, prior to launching a user-supplied
kernel loaded by a pv-grub stubdom.

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