KVM: VMX: Reset eVMCS controls in VP assist page during hardware disabling

Reset the eVMCS controls in the per-CPU VP assist page during hardware
disabling instead of waiting until kvm-intel's module exit.  The controls
are activated if and only if KVM creates a VM, i.e. don't need to be
reset if hardware is never enabled.

Doing the reset during hardware disabling will naturally fix a potential
NULL pointer deref bug once KVM disables CPU hotplug while enabling and
disabling hardware (which is necessary to fix a variety of bugs).  If the
kernel is running as the root partition, the VP assist page is unmapped
during CPU hot unplug, and so KVM's clearing of the eVMCS controls needs
to occur with CPU hot(un)plug disabled, otherwise KVM could attempt to
write to a CPU's VP assist page after it's unmapped.

Reported-by: Vitaly Kuznetsov <vkuznets@redhat.com>
Signed-off-by: Sean Christopherson <seanjc@google.com>
Reviewed-by: Vitaly Kuznetsov <vkuznets@redhat.com>
Message-Id: <20221130230934.1014142-11-seanjc@google.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>