Add a sysinfo util module and read host info API

Add a sysinfo util module and read host info API

Move existing routines about virSysinfoDef to an util module,
add a new entry point virSysinfoRead() to read the host values
with dmidecode

* src/conf/domain_conf.c src/conf/domain_conf.h src/util/sysinfo.c
  src/util/sysinfo.h: move to a new module, add virSysinfoRead()
* src/Makefile.am: handle the new module build
* src/libvirt_private.syms: new internal symbols
* include/libvirt/virterror.h src/util/virterror.c: defined a new
  error code for that module
* po/POTFILES.in: add new file for translations