virsh: Move command group definition into its own file

virsh: Move command group definition into its own file

* virsh-domain-monitor.c: Add domMonitoringCmds
* virsh-domain.c: Add domManagementCmds
* virsh-host.c: Add hostAndHypervisorCmds
* virsh-interface.c: Add ifaceCmds
* virsh-network.c: Add networkCmds
* virsh-nodedev.c: Add nodedevCmds
* virsh-nwfilter.c: Add nwfilterCmds
* virsh-pool.c: Add storagePoolCmds
* virsh-secret.c: Add secretCmds
* virsh-snapshot.c: Add snapshotCmds
* virsh-volume.c: Add storageVolCmds
* virsh.c: Remove all the above *Cmds.