viridian: make viridian_time_domain_freeze() safe to call...

viridian: make viridian_time_domain_freeze() safe to call...

...on a partially destroyed domain.

viridian_time_domain_freeze() and viridian_time_vcpu_freeze() rely
(respectively) on the dynamically allocated per-domain and per-vcpu viridian
areas [1], which are freed during domain_relinquish_resources().
Because arch_domain_pause() can call viridian_domain_time_freeze() this
can lead to host crashes if e.g. a XEN_DOMCTL_pausedomain is issued after
domain_relinquish_resources() has run.

To prevent such crashes, this patch adds a check of is_dying into
viridian_time_domain_freeze(), and viridian_time_domain_thaw() which is
similarly vulnerable to indirection into freed memory.

NOTE: The patch also makes viridian_time_vcpu_freeze/thaw() static, since
      they have no callers outside of the same source module.

[1] See commit e7a9b5e72f26 "viridian: separately allocate domain and vcpu
    structures".

Signed-off-by: Paul Durrant <paul.durrant@citrix.com>
Reviewed-by: Roger Pau Monné <roger.pau@citrix.com>