x86/msr: introduce guest_wrmsr()

x86/msr: introduce guest_wrmsr()

The new function is responsible for handling WRMSR from both HVM and PV
guests. Currently it handles only 2 MSRs:

    MSR_INTEL_PLATFORM_INFO
    MSR_INTEL_MISC_FEATURES_ENABLES

It has a different behaviour compared to the old MSR handlers: if MSR
is being handled by guest_wrmsr() then WRMSR will either succeed (if
a guest is allowed to access it and provided a correct value based on
its MSR policy) or produce a GP fault. A guest will never see
a successful WRMSR of some MSR unknown to this function.

guest_wrmsr() unifies and replaces the handling code from
vmx_msr_write_intercept() and priv_op_write_msr().

Signed-off-by: Sergey Dyasli <sergey.dyasli@citrix.com>
This also fixes a bug on AMD hardware where a guest which tries to
enable CPUID faulting via a direct write to the MSR will observe it
appearing to succeed, but because Xen actually ignored the write.

Reviewed-by: Andrew Cooper <andrew.cooper3@citrix.com>
Reviewed-by: Kevin Tian <kevin.tian@intel.com>