KVM: x86: Clean up handle_emulation_failure()

When handling emulation failure, return the emulation result directly
instead of capturing it in a local variable.  Future patches will move
additional cases into handle_emulation_failure(), clean up the cruft
before so there isn't an ugly mix of setting a local variable and
returning directly.

Reviewed-by: Vitaly Kuznetsov <vkuznets@redhat.com>
Reviewed-by: Liran Alon <liran.alon@oracle.com>
Signed-off-by: Sean Christopherson <sean.j.christopherson@intel.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>