KVM: VMX: Move Intel PT shenanigans out of VMXON/VMXOFF flows

Move the Intel PT tracking outside of the VMXON/VMXOFF helpers so that
a future patch can drop KVM's kvm_cpu_vmxoff() in favor of the kernel's
cpu_vmxoff() without an associated PT functional change, and without
losing symmetry between the VMXON and VMXOFF flows.

Barring undocumented behavior, this should have no meaningful effects
as Intel PT behavior does not interact with CR4.VMXE.

Signed-off-by: Sean Christopherson <seanjc@google.com>
Message-Id: <20201231002702.22237077-6-seanjc@google.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>