KVM: SVM: Use "standard" stgi() helper when disabling SVM

Now that kvm_rebooting is guaranteed to be true prior to disabling SVM
in an emergency, use the existing stgi() helper instead of open coding
STGI.  In effect, eat faults on STGI if and only if kvm_rebooting==true.

Link: https://lore.kernel.org/r/20230721201859.2307736-19-seanjc@google.com


Signed-off-by: Sean Christopherson <seanjc@google.com>