conf: move common disk source functions

conf: move common disk source functions

Move some functions out of domain_conf for use in the next
patch where snapshot starts to directly use structs in
virstoragefile.

* src/conf/domain_conf.c (virDomainDiskDefFree)
(virDomainDiskSourcePoolDefParse): Adjust callers.
(virDomainDiskSourceDefClear, virDomainDiskSourcePoolDefFree)
(virDomainDiskAuthClear): Move...
* src/util/virstoragefile.c (virStorageSourceClear)
(virStorageSourcePoolDefFree, virStorageSourceAuthClear): ...and
rename.
* src/conf/domain_conf.h (virDomainDiskAuthClear): Drop
declaration.
* src/qemu/qemu_conf.c (qemuTranslateDiskSourcePool): Adjust
caller.
* src/util/virstoragefile.h: Declare them.
* src/libvirt_private.syms (virstoragefile.h): Export them.

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