KVM: nVMX: Switch to vmcs01 to update SVI on-demand if L2 is active

KVM: nVMX: Switch to vmcs01 to update SVI on-demand if L2 is active

If APICv is activated while L2 is running and triggers an SVI update,
temporarily load vmcs01 and immediately update SVI instead of deferring
the update until the next nested VM-Exit.  This will eventually allow
killing off kvm_apic_update_hwapic_isr(), and all of nVMX's deferred
APICv updates.

Reviewed-by: Chao Gao <chao.gao@intel.com>
Link: https://patch.msgid.link/20260109034532.1012993-5-seanjc@google.com
Signed-off-by: Sean Christopherson <seanjc@google.com>