nested vmx: Validate host VMX MSRs before accessing them

nested vmx: Validate host VMX MSRs before accessing them

Some VMX MSRs may not exist on certain processor models, or may
be disabled because of configuration settings.   It is only safe to
access these MSRs if configuration flags in other MSRs are set.  These
prerequisites are listed in the Intel 64 and IA-32 Architectures
Software Developer’s Manual, Vol 3, Appendix A.

nvmx_msr_read_intercept() does not check the prerequisites before
accessing MSR_IA32_VMX_PROCBASED_CTLS2, MSR_IA32_VMX_EPT_VPID_CAP,
MSR_IA32_VMX_VMFUNC on the host.   Accessing these MSRs from a nested
VMX guest running on a host which does not support them will cause
Xen to crash with a GPF.

Signed-off-by: Euan Harris <euan.harris@citrix.com>
Acked-by: Kevin Tian <kevin.tian@intel.com>
Reviewed-by: Jan Beulich <jbeulich@suse.com>
Reviewed-by: Andrew Cooper <andrew.cooper3@citrix.com>