* include/libvirt/libvirt.h, include/libvirt/libvirt.h.in: Add
  virSecretGetUsageType, virSecretGetUsageID and virLookupSecretByUsage
* python/generator.py: Mark virSecretGetUsageType, virSecretGetUsageID
  as not throwing exceptions
* qemud/remote.c: Implement dispatch for virLookupSecretByUsage
* qemud/remote_protocol.x: Add usage type & ID as attributes of
  remote_nonnull_secret. Add RPC calls for new public APIs
* qemud/remote_dispatch_args.h, qemud/remote_dispatch_prototypes.h,
  qemud/remote_dispatch_ret.h, qemud/remote_dispatch_table.h,
  qemud/remote_protocol.c, qemud/remote_protocol.h: Re-generate
* src/datatypes.c, src/datatypes.h: Add usageType and usageID as
  properties of virSecretPtr
* src/driver.h: Add virLookupSecretByUsage driver entry point
* src/libvirt.c: Implement virSecretGetUsageType, virSecretGetUsageID
  and virLookupSecretByUsage
* src/libvirt_public.syms: Export virSecretGetUsageType, virSecretGetUsageID
  and virLookupSecretByUsage
* src/remote_internal.c: Implement virLookupSecretByUsage entry
* src/secret_conf.c, src/secret_conf.h: Remove the
  virSecretUsageType enum, now in public API. Make volume
  path mandatory when parsing XML
* src/secret_driver.c: Enforce usage uniqueness when defining secrets.
  Implement virSecretLookupByUsage api method
* src/virsh.c: Include usage for secret-list command