Encryption keys can be associated with each source file in a
backing chain; as such, this file belongs more in util/ where
it can be used by virstoragefile.h.

* src/conf/storage_encryption_conf.h: Rename...
* src/util/virstorageencryption.h: ...to this.
* src/conf/storage_encryption_conf.c: Rename...
* src/util/virstorageencryption.c: ...to this.
* src/Makefile.am (ENCRYPTION_CONF_SOURCES, CONF_SOURCES)
(UTIL_SOURCES): Update to new file names.
* src/libvirt_private.syms: Likewise.
* src/conf/domain_conf.h: Update client.
* src/conf/storage_conf.h: Likewise.
Signed-off-by: NEric Blake <eblake@redhat.com>