Add a VIR_SECRET_GET_VALUE_INTERNAL_CALL flag value, replacing the
originally separate libvirt_internal_call parameter.  The flag is used
to differentiate external virSecretGetValue() calls from internal calls
by libvirt drivers that need to use the secret even if it is private.

* src/libvirt_internal.h Remove VIR_DOMAIN_XML_FLAGS_MASK
* src/driver.h Add VIR_SECRET_GET_VALUE_FLAGS_MASK constant and
  VIR_SECRET_GET_VALUE_INTERNAL_CALL. Re-add the
  VIR_DOMAIN_XML_FLAGS_MASK constant
* src/libvirt.c (virSecretGetValue): Don't allow the user to specify
  internal flags.