xen/dom0: Fix latent dom0 construction bugs on all architectures

xen/dom0: Fix latent dom0 construction bugs on all architectures

 * x86 PV and ARM dom0's must not clear _VPF_down from v->pause_flags until
   all state is actually set up.  As it currently stands, d0v0 is eligible for
   scheduling before its registers have been set.  This is latent as we also
   hold a systemcontroller pause reference at the time which prevents d0 from
   being scheduled.

 * x86 PVH previously was not setting v->is_initialised for d0v0, despite
   setting the vcpu running eventually.  Therefore, a later VCPUOP_initialise
   hypercall will modify state under the feet of the running vcpu.  This is
   latent as PVH dom0 construction don't yet function.

Signed-off-by: Andrew Cooper <andrew.cooper3@citrix.com>
Reviewed-by: Jan Beulich <jbeulich@suse.com>
Reviewed-by: Stefano Stabellini <sstabellini@kernel.org>
Reviewed-by: Roger Pau Monné <roger.pau@citrix.com>
Release-acked-by: Julien Grall <julien.grall@linaro.org>