x86/reboot: KVM: Disable SVM during reboot via virt/KVM reboot callback

Use the virt callback to disable SVM (and set GIF=1) during an emergency
instead of blindly attempting to disable SVM.  Like the VMX case, if a
hypervisor, i.e. KVM, isn't loaded/active, SVM can't be in use.

Acked-by: Kai Huang <kai.huang@intel.com>
Link: https://lore.kernel.org/r/20230721201859.2307736-5-seanjc@google.com
Signed-off-by: Sean Christopherson <seanjc@google.com>