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>

qga/commands-posix.c

@@ -171,7 +171,7 @@ void qmp_guest_set_time(bool has_time, int64_t time_ns, Error **errp)
     /* Now, if user has passed a time to set and the system time is set, we
      * just need to synchronize the hardware clock. However, if no time was
      * passed, user is requesting the opposite: set the system time from the
-     * hardware clock. */
+     * hardware clock (RTC). */
     pid = fork();
     if (pid == 0) {
         setsid();

qga/qapi-schema.json

@@ -96,8 +96,8 @@
 ##
 # @guest-get-time:
 #
-# Get the information about guest time relative to the Epoch
-# of 1970-01-01 in UTC.
+# Get the information about guest's System Time relative to
+# the Epoch of 1970-01-01 in UTC.
 #
 # Returns: Time in nanoseconds.
 #
@@ -117,11 +117,11 @@
 # gap was, NTP might not be able to resynchronize the
 # guest.
 #
-# This command tries to set guest time to the given value,
-# then sets the Hardware Clock to the current System Time.
-# This will make it easier for a guest to resynchronize
-# without waiting for NTP. If no @time is specified, then
-# the time to set is read from RTC.
+# This command tries to set guest's System Time to the
+# given value, then sets the Hardware Clock (RTC) to the
+# current System Time. This will make it easier for a guest
+# to resynchronize without waiting for NTP. If no @time is
+# specified, then the time to set is read from RTC.
 #
 # @time: #optional time of nanoseconds, relative to the Epoch
 #        of 1970-01-01 in UTC.