virprocess: fix MinGW build and RHEL-5 build

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>