vircgroup: no need to ifdef virCgroupFree

vircgroup: no need to ifdef virCgroupFree

virCgroup struct is always defined and the free function is not calling
anything that would require OS supporting cgroups.

This fixes an issue if we try to start a VM with QEMU binary that
doesn't support QXL.  The start operation will fail in
qemuProcessStartValidateVideo() which will set correct error message,
but later in one of the cleanup paths we will call
qemuDomainObjPrivateDataClear() which always calls virCgroupFree()
and that will fail on OS that doesn't support cgroups and it will
set a new error which will be eventually reported to user.

Signed-off-by: Pavel Hrdina <phrdina@redhat.com>