qemu: Enable SCHED_CORE for vCPUs

qemu: Enable SCHED_CORE for vCPUs

For QEMU_SCHED_CORE_VCPUS case, the vCPU threads should be placed
all into one scheduling group, but not the emulator or any of its
threads. Therefore, as soon as vCPU TIDs are detected, fork off a
child which then creates a separate scheduling group and adds all
vCPU threads into it.

Please note, this commit only handles the cold boot case. Hotplug
is going to be implemented in the next commit.

Signed-off-by: Michal Privoznik <mprivozn@redhat.com>
Reviewed-by: Daniel P. Berrangé <berrange@redhat.com>