x86/hvm: Disable MPX by default

x86/hvm: Disable MPX by default

Memory Protection eXtension support has been dropped from GCC and Linux, and
will be dropped from future Intel CPUs.

With all other default/max pieces in place, move MPX from default to max.
This means that VMs won't be offered it by default, but can explicitly opt
into using it via cpuid="host,mpx=1" in their vm.cfg file.

The difference as visible to the guest is:

  diff --git a/default b/mpx
  index 0e91765d6b..c8c33cd584 100644
  --- a/default
  +++ b/mpx
  @@ -13,15 +13,17 @@ Native cpuid:
     00000004:00000004 -> 00000000:00000000:00000000:00000000
     00000005:ffffffff -> 00000000:00000000:00000000:00000000
     00000006:ffffffff -> 00000000:00000000:00000000:00000000
  -  00000007:00000000 -> 00000000:009c2fbb:00000000:9c000400
  +  00000007:00000000 -> 00000000:009c6fbb:00000000:9c000400
     00000008:ffffffff -> 00000000:00000000:00000000:00000000
     00000009:ffffffff -> 00000000:00000000:00000000:00000000
     0000000a:ffffffff -> 00000000:00000000:00000000:00000000
     0000000b:ffffffff -> 00000000:00000000:00000000:00000000
     0000000c:ffffffff -> 00000000:00000000:00000000:00000000
  -  0000000d:00000000 -> 00000007:00000240:00000340:00000000
  +  0000000d:00000000 -> 0000001f:00000240:00000440:00000000
     0000000d:00000001 -> 0000000f:00000240:00000000:00000000
     0000000d:00000002 -> 00000100:00000240:00000000:00000000
  +  0000000d:00000003 -> 00000040:000003c0:00000000:00000000
  +  0000000d:00000004 -> 00000040:00000400:00000000:00000000
     40000000:ffffffff -> 40000005:566e6558:65584d4d:4d4d566e
     40000001:ffffffff -> 0004000e:00000000:00000000:00000000
     40000002:ffffffff -> 00000001:40000000:00000000:00000000

Adjust the legacy restore path in libxc to cope safely with pre-4.14 VMs.

Signed-off-by: Andrew Cooper <andrew.cooper3@citrix.com>
Reviewed-by: Jan Beulich <jbeulich@suse.com>
Acked-by: Ian Jackson <ian.jackson@eu.citrix.com>
Release-acked-by: Paul Durrant <paul@xen.org>