storage: Add flags to allow building pool during create processing

storage: Add flags to allow building pool during create processing

https://bugzilla.redhat.com/show_bug.cgi?id=830056

Add flags handling to the virStoragePoolCreate and virStoragePoolCreateXML
API's which will allow the caller to provide the capability for the storage
pool create API's to also perform a pool build during creation rather than
requiring the additional buildPool step. This will allow transient pools
to be defined, built, and started.

The new flags are:

    * VIR_STORAGE_POOL_CREATE_WITH_BUILD
      Perform buildPool without any flags passed.

    * VIR_STORAGE_POOL_CREATE_WITH_BUILD_OVERWRITE
      Perform buildPool using VIR_STORAGE_POOL_BUILD_OVERWRITE flag.

    * VIR_STORAGE_POOL_CREATE_WITH_BUILD_NO_OVERWRITE
      Perform buildPool using VIR_STORAGE_POOL_BUILD_NO_OVERWRITE flag.

It is up to the backend to handle the processing of build flags. The
overwrite and no-overwrite flags are mutually exclusive.

NB:
This patch is loosely based upon code originally authored by Osier
Yang that were not reviewed and pushed, see:

https://www.redhat.com/archives/libvir-list/2012-July/msg01328.html