x86/pv: allow reading FEATURE_CONTROL MSR

x86/pv: allow reading FEATURE_CONTROL MSR

Linux PV guests will attempt to read the FEATURE_CONTROL MSR, so move
the handling done in VMX code into guest_rdmsr as it can be shared
between PV and HVM guests that way.

Note that there's a slight behavior change and attempting to read the
MSR when no features are available will result in a fault.

Signed-off-by: Roger Pau Monné <roger.pau@citrix.com>
Reviewed-by: Jan Beulich <jbeulich@suse.com>
Reviewed-by: Kevin Tian <kevin.tian@intel.com>