Description of problem:
A code cleanup to virsh ended up causing any command with an optional --domain parameter (virsh event, virsh qemu-monitor-event) to reject a domain argument with an error and proceed as if no domain filtering had been supplied.

Version-Release number of selected component (if applicable):
libvirt-1.2.14-1.fc22.x86_64

How reproducible:
100%

Steps to Reproduce:
1. virsh list --all
2. virsh event $dom1 (for some $dom in the list from step 1)
3. virsh start $dom2

Actual results:
error: internal error: virsh event: no domainVSH_OT_DATA option
then a startup event is listed for $dom2

Expected results:
events should be filtered to only $dom1

Additional info:
fixed by commit 31ef0836a73ed8583ff93613d2759e28965103ef
Author: Eric Blake <eblake>
Date:   Tue Apr 14 16:15:06 2015 -0600

    virsh: fix regression in 'virsh event' by domain
    
    Commit a0670ae caused a regression in 'virsh event' and
    'virsh qemu-monitor-event' - if a user tries to filter the
    command to a specific domain, an error message is printed:
    
    $ virsh event dom --loop
    error: internal error: virsh qemu-monitor-event: no domain VSH_OT_DATA option
    
    and then the command continues as though no domain had been
    supplied (giving events for ALL domains, instead of the
    requested one).  This is because the code was incorrectly
    assuming that all "domain" options would be supplied via a
    mandatory VSH_OT_DATA, even though "domain" is optional for
    these two commands, so we had changed them to VSH_OT_STRING
    to quit failing for other reasons (ever since it was decided
    that VSH_OT_DATA and VSH_OT_STRING should no longer be
    synonyms).
    
    In looking at the situation, though, the code for looking up
    a domain was making a pointless check for whether the option
    exists prior to finding the option's string value, as
    vshCommandOptStringReq does just fine at reporting any errors
    when looking up a string whether or not the option was present.
    
    So this is a case of regression fixing by pure code deletion :)
    
    * tools/virsh-domain.c (vshCommandOptDomainBy): Drop useless filter.
    * tools/virsh-interface.c (vshCommandOptInterfaceBy): Likewise.
    * tools/virsh-network.c (vshCommandOptNetworkBy): Likewise.
    * tools/virsh-nwfilter.c (vshCommandOptNWFilterBy): Likewise.
    * tools/virsh-secret.c (vshCommandOptSecret): Likewise.
    * tools/virsh.h (vshCmdHasOption): Drop unused function.
    * tools/virsh.c (vshCmdHasOption): Likewise.
    
    Signed-off-by: Eric Blake <eblake>