* include/libvirt/virterror.h, src/virterror.c: Add VIR_WAR_NO_SECRET
* src/libvirt_private.syms, src/datatypes.h, src/datatypes.c: Type
  virSecret struct definition and helper APIs
* src/driver.h: Sub-driver API definitions for secrets
* src/libvirt.c: Define new sub-driver for secrets