xen: Plumb an is_priv boolean into domain_create()

xen: Plumb an is_priv boolean into domain_create()

The current mechanism of setting dom0->is_privileged after construction means
that the is_control_domain() predicate returns false during construction.

In particular, this means that the CPUID Faulting special case in
init_domain_msr_policy() fails to take effect.  (In actual fact, faulting
support is advertised to dom0, but attempting to configure it is silently
ignored because of the dom0 special case in ctxt_switch_levelling().)

This could be implemented using a flag in xen_domctl_createdomain, but using
an extra boolean parameter like this means that we can't accidentally allow
domain_create() to create a second dom0 due to parameter mis-auditing.

While adjusting the setting of dom0->is_privileged, drop the redundant zeroing
of dom0->target.

Signed-off-by: Andrew Cooper <andrew.cooper3@citrix.com>
Reviewed-by: Jan Beulich <jbeulich@suse.com>
Reviewed-by: Wei Liu <wei.liu2@citrix.com>
Reviewed-by: Roger Pau Monné <roger.pau@citrix.com>
Acked-by: Julien Grall <julien.grall@arm.com>