Revert "maint: prefer enum over int for virstoragefile structs"
    
    This partially reverts commits b279e52f7 and ea18f8b2.
    
    It turns out our code base is full of:
    
    if ((struct.member = virBlahFromString(str)) < 0)
        goto error;
    
    Meanwhile, the C standard says it is up to the compiler whether
    an enum is signed or unsigned when all of its declared values
    happen to be positive.  In my testing (Fedora 20, gcc 4.8.2),
    the compiler picked signed, and nothing changed.  But others
    testing with gcc 4.7 got compiler warnings, because it picked
    the enum to be unsigned, but no unsigned value is less than 0.
    Even worse:
    
    if ((struct.member = virBlahFromString(str)) <= 0)
        goto error;
    
    is silently compiled without warning, but incorrectly treats -1
    from a bad parse as a large positive number with no warning; and
    without the compiler's help to find these instances, it is a
    nightmare to maintain correctly.  We could force signed enums
    with a dummy negative declaration in each enum, or cast the
    result of virBlahFromString back to int after assigning to an
    enum value, or use a temporary int for collecting results from
    virBlahFromString, but those actions are all uglier than what we
    were trying to cure by directly using enum types for struct
    values in the first place.  It's better off to just live with int
    members, and use 'switch ((virFoo) struct.member)' where we want
    the compiler to help, than to track down all the conversions from
    string to enum and ensure they don't suffer from type problems.
    
    * src/util/virstorageencryption.h: Revert back to int declarations
    with comment about enum usage.
    * src/util/virstoragefile.h: Likewise.
    * src/conf/domain_conf.c: Restore back to casts in switches.
    * src/qemu/qemu_driver.c: Likewise.
    * src/qemu/qemu_command.c: Add cast rather than revert.
    
    Signed-off-by: Eric Blake <eblake@redhat.com>