Merge the duplicated code for create, destroy, migrate, sysrq, and shutdown

Merge the duplicated code for create, destroy, migrate, sysrq, and shutdown
into one handler that dynamically imports the correct subcommand handler.
Remove the fullhelp cruft, which was referencing something not actually present,
and was unused.  Remove xm_network_attach and xm_network_detach, which were
providing code for unadvertised and unsupported functionality.

Signed-off-by: Ewan Mellor <ewan@xensource.com>