ANDROID: pkvm: x86: Fix NULL deref in sync_shadow_context_entry()

Fix possible NULL dereference of guest_ce when legacy IOMMU mode is
used. This NULL dereference happens when a shadow context entry is
freed, for instance during the machine reboot, when the host calls
intel_disable_iommus().

Bug: 395299836
Test: Boot, verify cpus are de-privileged and run a minimal protected vm.

Change-Id: I86f00855e838fbc079a28144f38b885eac07b166
Signed-off-by: Dmytro Maluka <dmy@semihalf.com>
Signed-off-by: Vineeth Pillai <vineethrp@google.com>