virprocess: fix MinGW build and RHEL-5 build
    
    Commit b6a2828e introduced new functions to set process scheduler. There
    is a small typo in ELSE path for systems where scheduler is not
    available.
    
    Also some of the definitions were introduced later in kernel. For
    example RHEL-5 is running on kernel 2.6.18, but SCHED_IDLE was introduces
    in 2.6.23 [1] and SCHED_BATCH in 2.6.16 [1]. We should not count only on
    existence of function sched_setscheduler(), we must also check for
    existence of used macros as they might not be defined.
    
    [1] see 'man 7 sched'
    
    Signed-off-by: Pavel Hrdina <phrdina@redhat.com>