x86/msr: introduce guest_rdmsr()

x86/msr: introduce guest_rdmsr()

The new function is responsible for handling RDMSR 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_rdmsr() then RDMSR will either succeed (if
a guest is allowed to access it based on its MSR policy) or produce
a GP fault. A guest will never see a H/W value of some MSR unknown to
this function.

guest_rdmsr() unifies and replaces the handling code from
vmx_msr_read_intercept() and priv_op_read_msr().

Signed-off-by: Sergey Dyasli <sergey.dyasli@citrix.com>
This (along with the prep work in init_domain_msr_policy()) also fixes
a bug where Dom0 could probe and find CPUID faulting, even though it
couldn't actually use it.

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