virsh: optimize creation of default connection

Ramon de Carvalho Valle reported a problem with:
virsh connect qemu:///system
as a non-root user.  The real root problem appears to be a regression
in libvirtd being auto-started on the default qemu:///session URI;
however, the symptom points to an independent flaw in virsh - we
shouldn't be wasting efforts on making a connection if we aren't going
to be using that connection.  Fixing virsh avoids Ramon's issue, while
I work in the meantime to fix the real libvirtd regression.

This patch looks big, but that's because 'gcc -Wmissing-field-initializers'
gets triggered by './autobuild.sh --enable-compile-warnings=error', so I
had to add 0 initialization to everyone (rather than my preference of
just adding the non-zero flags to virshCmds and to cmdConnect).

Meanwhile, if you use 'virsh -c URI', the connection must succeed; this
patch _only_ optimizes the default connection to be deferred to a later
point where we know if a particular command to be run needs a connection.

* tools/virsh.c (VSH_CMD_FLAG_NOCONNECT): New flag.
(vshCmdDef): Add new flags field.
(vshCommandRun): Honor new flag.
(domManagementCmds, domMonitoringCmds, storagePoolCmds)
(storageVolCmds, networkCmds, nodedevCmds, ifaceCmds)
(nwfilterCmds, secretCmds, virshCmds, snapshotCmds)
(hostAndHypervisorCmds): Populate new field.
(vshReconnect): Don't warn on initial connection.