KVM: selftests: map Xen's shared_info page using HVA rather than GFN

KVM: selftests: map Xen's shared_info page using HVA rather than GFN

Using the HVA of the shared_info page is more efficient, so if the
capability (KVM_XEN_HVM_CONFIG_SHARED_INFO_HVA) is present use that method
to do the mapping.

NOTE: Have the juggle_shinfo_state() thread map and unmap using both
      GFN and HVA, to make sure the older mechanism is not broken.

Signed-off-by: Paul Durrant <pdurrant@amazon.com>
Reviewed-by: David Woodhouse <dwmw@amazon.co.uk>
---
Cc: Sean Christopherson <seanjc@google.com>
Cc: Paolo Bonzini <pbonzini@redhat.com>
Cc: David Woodhouse <dwmw2@infradead.org>
v13:
 - Patch title change.

v3:
 - Re-work the juggle_shinfo_state() thread.

v2:
 - New in this version.