Currently, qmp_guest_set_time() sets system time using
the settimeofday(2) system call and then runs the hwclock(8)
util to set hardware time.

hwclock(8) is Linux only, so current implementation
always fails while trying to run it on non-Linux POSIX OSes,
such as BSDs (FreeBSD, OpenBSD, NetBSD) and illumos.

Update the code not to run it on non-Linux. The settimeofday(2)
call sets both system and hardware time on these systems.
For example, on FreeBSD settimeofday() calls settime()
which calls resettodr() [1] which updates RTC clock [2].
It is similar on OpenBSD and NetBSD, though implementations
differ a bit.

Additional detail: on FreeBSD RTC clock setting can be
disabled with the `machdep.disable_rtc_set` sysctl.
As we cannot do anything about it, do not handle that
case separately.

Non-Linux POSIX OSes do not provide a common utility
equivalent to hwclock -s, so report an unsupported operation
when no time is supplied.

1: https://cgit.freebsd.org/src/tree/sys/kern/kern_time.c
2: https://cgit.freebsd.org/src/tree/sys/kern/subr_rtc.c

Signed-off-by: Roman Bogorodskiy <bogorodskiy@gmail.com>
---
Changes since v1:

Replace "#if defined(CONFIG_BSD)" condition with
"#if !defined(CONFIG_LINUX)" as hwclock(8) is Linux only.

 qga/commands-posix.c | 14 ++++++++++++++
 1 file changed, 14 insertions(+)

diff --git a/qga/commands-posix.c b/qga/commands-posix.c
index 837e285b6d..ffd1744b74 100644
--- a/qga/commands-posix.c
+++ b/qga/commands-posix.c
@@ -322,6 +322,20 @@ void qmp_guest_set_time(bool has_time, int64_t time_ns, Error **errp)
         }
     }
 
+#if !defined(CONFIG_LINUX)
+    /*
+     * BSDs (FreeBSD, OpenBSD, NetBSD) and illumos do not have the
+     * hwclock(8) tool. These systems update hardware clock when
+     * settimeofday() sets system time, so no additional actions
+     * are needed. Setting system time from RTC is not supported.
+     */
+    if (!has_time) {
+        error_setg(errp, "Setting system time from RTC is not supported");
+    }
+
+    return;
+#endif
+
     /* 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
-- 
2.55.0