virsh: fix regression in 'virsh event' by domain

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@redhat.com>