KVM: nVMX: introduce struct nested_vmx_msrs

Move the MSRs to a separate struct, so that we can introduce a global
instance and return it from the /dev/kvm KVM_GET_MSRS ioctl.

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