qemu: Add qemuProcessSetupPid() and use it in qemuProcessSetupIOThread()

qemu: Add qemuProcessSetupPid() and use it in qemuProcessSetupIOThread()

Setting up cgroups and other things for all kinds of threads (the
emulator thread, vCPU threads, I/O threads) was copy-pasted every time
new thing was added.  Over time each one of those functions changed a
bit differently.  So create one function that does all that setup and
start using it, starting with I/O thread setup.  That will shave some
duplicated code and maybe fix some bugs as well.

Signed-off-by: Martin Kletzander <mkletzan@redhat.com>