qemu: Substract isolcpus from all online affinity

When starting a domain and there's no vCPU/emulator pinning set,
we query the list of all online physical CPUs and set affinity of
the child process (which eventually becomes QEMU) to that list.
We can't assume libvirtd itself had affinity to all online CPUs
and since affinity of the child process is inherited, we should
fix it afterwards. But that's not necessarily correct. Users
might isolate some physical CPUs and we should avoid touching
them unless explicitly told so (i.e. vCPU/emulator pinning told
us so).

Therefore, when attempting to set affinity to all online CPUs
subtract the isolated ones.

Before this commit:

  root@localhost:~# cat /sys/devices/system/cpu/isolated
  19,21,23
  root@virtlab414:~# taskset -cp $(pgrep qemu)
  pid 14835's current affinity list: 0-23

After:

  root@virtlab414:~# taskset -cp $(pgrep qemu)
  pid 17153's current affinity list: 0-18,20,22

Resolves: https://issues.redhat.com/browse/RHEL-33082


Signed-off-by: Michal Privoznik <mprivozn@redhat.com>
Reviewed-by: Pavel Hrdina <phrdina@redhat.com>