cpus.c: ensure running CPU recalculates icount deadlines on timer expiry
    
    When we run in TCG icount mode, we calculate the number of instructions
    to execute using tcg_get_icount_limit(), which ensures that we stop
    execution at the next timer deadline. However there is a bug where
    currently we do not recalculate that limit if the guest reprograms
    a timer so that the next deadline moves closer, and so we will
    continue execution until the original limit and fire the timer
    later than we should.
    
    Fix this bug in qemu_timer_notify_cb(): if we are currently running
    a VCPU in icount mode, we simply need to kick it out of the main
    loop and back to tcg_cpu_exec(), where it will recalculate the
    icount limit. If we are not currently running a VCPU, then we
    retain the existing logic for waking up a halted CPU.
    
    Cc: qemu-stable@nongnu.org
    Fixes: https://bugs.launchpad.net/qemu/+bug/1754038
    Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
    Reviewed-by: Richard Henderson <richard.henderson@linaro.org>
    Reviewed-by: Alex Bennée <alex.bennee@linaro.org>
    Message-id: 20180406123838.21249-1-peter.maydell@linaro.org
    (cherry picked from commit c52e7132d7c885841500f5277f7305f62767fe1d)
    Signed-off-by: Michael Roth <mdroth@linux.vnet.ibm.com>