KVM: nVMX: list VMX MSRs in KVM_GET_MSR_INDEX_LIST

This allows userspace to know which MSRs are supported by the hypervisor.
Unfortunately userspace must resort to tricks for everything except
MSR_IA32_VMX_VMFUNC (which was just added in the previous patch).
One possibility is to use the feature control MSR, which is tied to nested
VMX as well and is present on all KVM versions that support feature MSRs.

Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>