public/xen.h: update the comment explaining 'Wallclock time'

public/xen.h: update the comment explaining 'Wallclock time'

Since commit 0629adfd80e "Actually set a HVM domain's time offset when it
sets the RTC", the comment in the public header has been misleading, since
it claims that wallclock time is only updated by control software.
Moreover, the comments stating that wc_sec and wc_nsec are seconds and
nanoseconds (respectively) in UTC since the Unix epoch are bogus. Their
values are adjusted by the domain's time_offset_seconds value, which is
updated by a guest write to the emulated RTC and hence the wallclock
timezone is under guest control.

This patch attempts to bring the comment in line with reality whilst
keeping it reasonably short.

Signed-off-by: Paul Durrant <paul.durrant@citrix.com>
Acked-by: Jan Beulich <jbeulich@suse.com>