x86/xen: print a warning when HVMOP_get_mem_type fails

HVMOP_get_mem_type is not expected to fail, "This call failing is
indication of something going quite wrong and it would be good to know
about this." [1]

Let's add a pr_warn_once().

Link: https://lkml.kernel.org/r/3b935aa0-6d85-0bcd-100e-15098add3c4c@oracle.com [1]
Link: https://lkml.kernel.org/r/20211005121430.30136-4-david@redhat.com


Signed-off-by: David Hildenbrand <david@redhat.com>
Suggested-by: Boris Ostrovsky <boris.ostrovsky@oracle.com>
Cc: Baoquan He <bhe@redhat.com>
Cc: Borislav Petkov <bp@alien8.de>
Cc: Dave Young <dyoung@redhat.com>
Cc: "H. Peter Anvin" <hpa@zytor.com>
Cc: Ingo Molnar <mingo@redhat.com>
Cc: Jason Wang <jasowang@redhat.com>
Cc: Juergen Gross <jgross@suse.com>
Cc: "Michael S. Tsirkin" <mst@redhat.com>
Cc: Michal Hocko <mhocko@suse.com>
Cc: Mike Rapoport <rppt@kernel.org>
Cc: Oscar Salvador <osalvador@suse.de>
Cc: "Rafael J. Wysocki" <rafael.j.wysocki@intel.com>
Cc: Stefano Stabellini <sstabellini@kernel.org>
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: Vivek Goyal <vgoyal@redhat.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>