libx86: introduce a helper to deserialise msr_policy objects

libx86: introduce a helper to deserialise msr_policy objects

As with the serialise side, Xen's copy_from_guest API is used, with a
compatibility wrapper for the userspace build.

Signed-off-by: Sergey Dyasli <sergey.dyasli@citrix.com>
Signed-off-by: Roger Pau Monné <roger.pau@citrix.com>
Signed-off-by: Andrew Cooper <andrew.cooper3@citrix.com>
Reviewed-by: Wei Liu <wei.liu2@citrix.com>
Reviewed-by: Jan Beulich <jbeulich@suse.com>
master commit: afec08b92ffe8b85d2bf2e8c7c221b63ba96743e
master date: 2019-03-12 14:12:23 +0000