x86/hvm/viridian: fix APIC assist page leak

x86/hvm/viridian: fix APIC assist page leak

Commit a6f2cdb6 "keep APIC assist page mapped..." introduced a page
leak because it relied on viridian_vcpu_deinit() always being called
to release the page mapping. This does not happen in the case a normal
domain shutdown.

This patch fixes the problem by introducing a new function,
viridian_domain_deinit(), which will iterate through the vCPUs and
release any page mappings still present.

Signed-off-by: Paul Durrant <paul.durrant@citrix.com>
Reviewed-by: Jan Beulich <jbeulich@suse.com>