KVM: VMX: Use the kernel's version of VMXOFF

Drop kvm_cpu_vmxoff() in favor of the kernel's cpu_vmxoff().  Modify the
latter to return -EIO on fault so that KVM can invoke
kvm_spurious_fault() when appropriate.  In addition to the obvious code
reuse, dropping kvm_cpu_vmxoff() also eliminates VMX's last usage of the
__ex()/__kvm_handle_fault_on_reboot() macros, thus helping pave the way
toward dropping them entirely.

Signed-off-by: Sean Christopherson <seanjc@google.com>
Message-Id: <20201231002702.22237077-7-seanjc@google.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>