docs: fix list of domain states in virsh manpage
    
    The virsh manpage lists "shutdown" and "dying" as two of the possible
    domain states that could be listed in the output of the "virsh list"
    command. However, a domain that is being shutdown will be listed as
    "in shutdown", and the "dying" state doesn't even exist (and never
    has, as far as I can tell from looking through git history - it was
    shown in the original import of the virsh.pod file in 2006; there was
    no VIR_DOMAIN_DYING state then, there wasn't one when those lines of
    virsh.pod were tweaked in 2008, and there still isn't one
    today. Apparently it was just something that sounded like a good idea
    to someone at some time, but was never implemented...)
    
    Resolves: https://bugzilla.redhat.com/1408778