KVM: x86: clarify leave_smm() return value

KVM: x86: clarify leave_smm() return value

The return value of vmx_leave_smm() is unrelated from that of
nested_vmx_enter_non_root_mode().  Check explicitly for success
(which happens to be 0) and return 1 just like everywhere
else in vmx_leave_smm().

Likewise, in svm_leave_smm() return 0/1 instead of the 0/1/-errno
returned by tenter_svm_guest_mode().

Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>