KVM: VMX: vmx_set_cr0 expects kvm->srcu locked

vmx_set_cr0 is called from vcpu run context, therefore it expects
kvm->srcu to be held (for setting up the real-mode TSS).

Signed-off-by: Marcelo Tosatti <mtosatti@redhat.com>
Signed-off-by: Avi Kivity <avi@redhat.com>