qemu: Reject unsupported tuning in session mode

qemu: Reject unsupported tuning in session mode

When domain is started with setting that cannot be done, i.e. those
that require cgroups, there is no error reported and it succeeds
without any message whatsoever.

When setting with API, virsh, an error is reported, but only due to
the fact that no cgroups are mounted (priv->cgroup == NULL).

Given the above it seems reasonable to reject such unsupported
settings.

This patch effectively changes the error message from:

$ virsh -c qemu:///session schedinfo dummy
Scheduler      : Unknown
error: Requested operation is not valid: cgroup CPU controller is not mounted

to:

$ virsh -c qemu:///session schedinfo dummy
Scheduler      : Unknown
error: Operation not supported: CPU tuning is not available in session mode

Resolves: https://bugzilla.redhat.com/show_bug.cgi?id=1023366

Signed-off-by: Martin Kletzander <mkletzan@redhat.com>