conf: use typedefs for enums in "src/conf/storage_conf.h"
    
    In "src/conf/" there are many enumeration (enum) declarations.
    Similar to the recent cleanup to "src/util" directory, it's
    better to use a typedef for variable types, function types and
    other usages. Other enumeration and folders will be changed to
    typedef's in the future. Most of the files changed in this
    commit are related to storage (storage_conf) enums.
    
    Signed-off-by: Julio Faracco <jcfaracco@gmail.com>
    Signed-off-by: Eric Blake <eblake@redhat.com>