conf: move source pool type to util/

conf: move source pool type to util/

Another struct being moved to util.  This one doesn't have as
much use yet, thankfully.

* src/conf/domain_conf.h (virDomainDiskSourcePoolMode)
(virDomainDiskSourcePoolDef): Move...
* src/util/virstoragefile.h (virStorageSourcePoolMode)
(virStorageSourcePoolDef): ...and rename.
* src/conf/domain_conf.c (virDomainDiskSourcePoolDefFree)
(virDomainDiskSourceDefClear, virDomainDiskSourcePoolDefParse)
(virDomainDiskDefParseXML, virDomainDiskSourceDefParse)
(virDomainDiskSourceDefFormatInternal)
(virDomainDiskDefForeachPath, virDomainDiskSourceIsBlockType):
Adjust clients.
* src/qemu/qemu_conf.c (qemuTranslateDiskSourcePool): Likewise.
* src/libvirt_private.syms (domain_conf.h): Move symbols...
(virstoragefile.h): ...as appropriate.

Signed-off-by: Eric Blake <eblake@redhat.com>