KVM: nVMX: don't clear mtf_pending when nested events are blocked

If nested events are blocked, don't clear the mtf_pending flag to avoid
missing later delivery of the MTF VM-exit.

Fixes: 5ef8acbd

 ("KVM: nVMX: Emulate MTF when performing instruction emulation")
Signed-off-by: Oliver Upton <oupton@google.com>
Message-Id: <20200406201237.178725-1-oupton@google.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>