audit: use bool for audit log choice
    
    We weren't ever using the value for anything other than being non-zero.
    
    * src/util/viraudit.h (virAuditLog): Change signature.
    * src/util/viraudit.c (virAuditLog): Update user.
    * daemon/libvirtd.c (main): Likewise.
    
    Signed-off-by: Eric Blake <eblake@redhat.com>