Virsh uses an error handler to save errors from libvirt. On some code
paths it's needed to clear libvirt errors and continue on fallback code
paths without reporting failure.

This patch adds function vshResetLibvirtError() that clears error
returned by libvirt and updates all places where the old two-line method
was used.

* 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.

This splits commands of storage pool group into virsh-pool.c,
The helpers not for common use are moved too. Standard copyright
is added for the new file.

* tools/virsh.c:
  Remove commands for storage storage pool and a few helpers.
  (vshCommandOptVol, vshCommandOptVolBy).

* tools/virsh-pool.c:
  New file, filled with commands of storage pool group and its
  helpers.

* po/POTFILES.in:
  Add virsh-pool.c

* cfg.mk:
  Skip to check config.h including for virsh-pool.c