virsh: Add build flags to pool-create[-as] and pool-start

virsh: Add build flags to pool-create[-as] and pool-start

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

Utilize recently added VIR_STORAGE_POOL_CREATE_WITH_BUILD* flags in
order to pass the flags along to the virStoragePoolCreateXML and
virStoragePoolCreate API's.

This affects the 'virsh pool-create', 'virsh pool-create-as', and
'virsh pool-start' commands.  While it could be argued that pool-start
doesn't need the flags, they could prove useful for someone trying to
do one command build --overwrite and start command processing or
essentially starting with a clean slate.

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/msg00497.html