docs: man: virsh: Document more carefully that 'guestinfo' can return nothing

docs: man: virsh: Document more carefully that 'guestinfo' can return nothing

When invoking 'virsh guestinfo $VM' without explicitly specifying a
group of information to return, virsh always reports success even when
the guest agent doesn't report any information in the current state.
This is desired in situations when you are okay with stats being missing
and avoids spurious errors being reported.

Clarify that this is really desired in the man page.

Resolves: https://bugzilla.redhat.com/show_bug.cgi?id=2041665
Signed-off-by: Peter Krempa <pkrempa@redhat.com>
Reviewed-by: Andrea Bolognani <abologna@redhat.com>
Signed-off-by: Peter Krempa <pkrempa@redhat.com>