kvm: x86: fix setting IA32_FEATURE_CONTROL with nested VMX disabled

This patch is to fix the bug https://bugs.launchpad.net/qemu-kvm/+bug/1207623

IA32_FEATURE_CONTROL is pointless if not expose VMX or SMX bits to
cpuid.1.ecx of vcpu. Current qemu-kvm will error return when kvm_put_msrs
or kvm_get_msrs.

Signed-off-by: Liu Jinsong <jinsong.liu@intel.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
