qga: trivial fix for unclear documentation of guest-set-time

qga: trivial fix for unclear documentation of guest-set-time

We mixed the use of "guest time", "system time", "hardware time",
"RTC" in documentation, it's unclear.

This patch just added two remarks of RTC and replace two "guest time"
by "guest's system time".

Signed-off-by: Amos Kong <akong@redhat.com>
Reviewed-by: Michal Privoznik <mprivozn@redhat.com>
Reviewed-by: Eric Blake <eblake@redhat.com>
Signed-off-by: Michael Tokarev <mjt@tls.msk.ru>