KVM: VMX: Save HOST_CR3 in vmx_prepare_switch_to_guest()

The host CR3 in the vcpu thread can only be changed when scheduling.
Moving the code in vmx_prepare_switch_to_guest() makes the code
simpler.

Signed-off-by: Lai Jiangshan <laijs@linux.alibaba.com>
Message-Id: <20211118110814.2568-5-jiangshanlai@gmail.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>