virsh: Categorize commands into groups for virsh help

Change the virsh help out. The new output of "virsh help" and
"virsh --help" will be like:

 Secret (help keyword 'secret'):
    secret-define                  define or modify a secret from an XML file
    secret-dumpxml                 secret attributes in XML
    secret-set-value               set a secret value
    secret-get-value               Output a secret value
    secret-undefine                undefine a secret
    secret-list                    list secrets

 Snapshot (help keyword 'snapshot'):
    snapshot-create                Create a snapshot
    snapshot-current               Get the current snapshot
    snapshot-delete                Delete a domain snapshot
    snapshot-dumpxml               Dump XML for a domain snapshot
    snapshot-list                  List snapshots for a domain
    snapshot-revert                Revert a domain to a snapshot

Also support output help information of specified command group, e.g.

% ./tools/virsh help "Network Filter"
 Network Filter (help keyword 'filter'):
    nwfilter-define                define or update a network filter from an XML file
    nwfilter-undefine              undefine a network filter
    nwfilter-dumpxml               network filter information in XML
    nwfilter-list                  list network filters
    nwfilter-edit                  edit XML configuration for a network filter

Each group has a help keyword, e.g.

% ./tools/virsh help filter
 Network Filter (help keyword 'filter'):
    nwfilter-define                define or update a network filter from an XML file
    nwfilter-undefine              undefine a network filter
    nwfilter-dumpxml               network filter information in XML
    nwfilter-list                  list network filters
    nwfilter-edit                  edit XML configuration for a network filter

* tools/virsh.c:
  - introduce new struct "vshCmdGrp" and macros to define the groups.
  - split previous array "commands" into small arrays which are orgnized
    by group
  - changed some functions, e.g. "vshCmdDefSearch"
  - Added new functions, e.g. "vshCmdGrpSearch"
  - commands of each group are in "alphabetical order" now.
  - command groups are in "alphabetical order" now.
  - the commands are categorized with reference of
    http://wiki.libvirt.org/page/VirshHelpV2 (by Justin)
  - the modifications doesn't affect tests

* TODO:
  - doc