qga: fence guest-set-time if hwclock not available

qga: fence guest-set-time if hwclock not available

The Posix implementation of guest-set-time invokes hwclock to
set/retrieve the time to/from the hardware clock. If hwclock
is not available, the user is currently informed that "hwclock
failed to set hardware clock to system time", which is quite
misleading. This may happen e.g. on s390x, which has a different
timekeeping concept anyway.

Let's check for the availability of the hwclock command and
return QERR_UNSUPPORTED for guest-set-time if it is not available.

Reviewed-by: Laszlo Ersek <lersek@redhat.com>
Reviewed-by: Daniel P. Berrangé <berrange@redhat.com>
Reviewed-by: Michael Roth <mdroth@linux.vnet.ibm.com>
Message-Id: <20191205115350.18713-1-cohuck@redhat.com>
Reviewed-by: Philippe Mathieu-Daudé <philmd@redhat.com>
Signed-off-by: Cornelia Huck <cohuck@redhat.com>