storage_encryption: silence clang warning

storage_encryption: silence clang warning

For printf("%*s",foo,bar), clang complains if foo is not int:

warning: field width should have type 'int', but argument has
type 'unsigned int' [-Wformat]

* src/conf/storage_encryption_conf.c
(virStorageEncryptionSecretFormat, virStorageEncryptionFormat):
Use correct type.
* src/conf/storage_encryption_conf.h (virStorageEncryptionFormat):
Likewise.