fix rtc-td-hack on host without high-res timers

On hosts without high-res timers it is impossible to inject rtc interrupt
faster then 1kHz. Windows sometimes configures RTC to generate 1kHz
interrupts, so we can't inject missed interrupts when running on such
hosts. Always injecting an interrupt on REG_C read is also not an option
since Windows wait for REG_C to become zero with interrupt disabled
during boot. This patch uses mixed approach: accelerate timer + inject
up to 1000 interrupts on REG_C read.

Signed-off-by: Gleb Natapov <gleb@redhat.com>
Signed-off-by: Anthony Liguori <aliguori@us.ibm.com>
