* src/internal.h: move xstrol() variants from here ...

* src/util.[ch]: ... to here and rename to virStrToLong()

* src/libvirt_sym.version: export __virStrToLong_i() for
virsh and qemud.

* src/nodeinfo.c, src/stats_linux.c, src/virsh.c,
  src/xend_internal.c, qemud/qemud.c: replace xstrtol()
calls with virStrToLong()

* src/nodeinfo.h: don't include internal.h, which was only
needed for xstrtol(), but instead include libvirt.h which
is suffificient for the declarations in the header.