hpet: fix infinite loop in qemu_run_timers with -icount enabled

hpet_timer timer callback rearms itself based on difference between
current HPET tick counter and comparator value. Difference calculated by
the hpet_calculate_diff function is limited to non-negative values.

cur_tick is calculated via hpet_get_ticks that uses qemu_get_clock_ns(vm_clock).

With -icount enabled vm_clock doesn't advance during qemu_run_timers
loop thus once difference is zero, qemu_run_timers loops forever
handling hpet_timer.

Limit hpet_calculate_diff results to positive only values to avoid that
infinite loop.

This fixes the following qemu-system-x86_64 hang when it reaches
timer_irq_works() in the linux bootup:

[    0.000000] Fast TSC calibration using PIT
[    0.000000] Detected 1000.054 MHz processor.
[    0.000031] Calibrating delay loop (skipped), value calculated using timer frequency.. 2000.10 BogoMIPS (lpj=10000540)
[    0.000404] pid_max: default: 32768 minimum: 301
[    0.001138] Mount-cache hash table entries: 256
[    0.003883] Initializing cgroup subsys ns
[    0.004035] Initializing cgroup subsys cpuacct
[    0.004280] Initializing cgroup subsys freezer
[    0.004790] Performance Events: AMD PMU driver.
[    0.004985] ... version:                0
[    0.005134] ... bit width:              48
[    0.005285] ... generic registers:      4
[    0.005437] ... value mask:             0000ffffffffffff
[    0.005625] ... max period:             00007fffffffffff
[    0.005807] ... fixed-purpose events:   0
[    0.005957] ... event mask:             000000000000000f
[    0.006275] SMP alternatives: switching to UP code

Signed-off-by: Max Filippov <jcmvbkbc@gmail.com>
Signed-off-by: Anthony Liguori <aliguori@us.ibm.com>
