In preparation for adding a MTTCG thread we re-factor out a bit of what
will be common code to handle the QEMU_CLOCK_VIRTUAL expiration.

Signed-off-by: Alex Bennée <alex.bennee@linaro.org>
Reviewed-by: Richard Henderson <rth@twiddle.net>
Message-Id: <20161027151030.20863-18-alex.bennee@linaro.org>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>