KVM: x86: Handle emulation failure directly in kvm_task_switch()

Consolidate the reporting of emulation failure into kvm_task_switch()
so that it can return EMULATE_USER_EXIT.  This helps pave the way for
removing EMULATE_FAIL altogether.

This also fixes a theoretical bug where task switch interception could
suppress an EMULATE_USER_EXIT return.

Signed-off-by: Sean Christopherson <sean.j.christopherson@intel.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>