cpus: fix TCG kick timer leak

This is an alternative fix to Marc-André's original patch.

Reported-by: Marc-André Lureau <marcandre.lureau@redhat.com>
Suggested-by: Paolo Bonzini <pbonzini@redhat.com>
Signed-off-by: Alex Bennée <alex.bennee@linaro.org>
Message-Id: <20180927171724.30128-1-alex.bennee@linaro.org>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>