[linux-xenclient-dom0] enable and use HRTIMERs for better sound

Without stubdomains, ioemu talks directly to ALSA, which is too slow when using regular timers at 100HZ.
Using hrtimers speeds up ALSA, and therefore ioemu.

Singed-off-by: Jed Lejosne <jed.openxt@gmail.com>