Add hook utilities

Add hook utilities

This exports 3 basic routines:
  - virHookInitialize() initializing the hook support by looking for
    scripts availability
  - virHookPresent() used to test if there is a hook for a given driver
  - virHookCall() which actually calls a synchronous script hook with
    the needed parameters
Note that this doesn't expose any public API except for the locations
and arguments passed to the scripts

* src/Makefile.am: add the 2 new files
* src/util/hooks.h src/util/hooks.c: implements the 3 functions
* src/libvirt_private.syms: export the 3 symbols internally
* po/POTFILES.in: add src/util/hooks.c to translatables modules