qemu: Move cpuset preference evaluation into a separate function

qemu: Move cpuset preference evaluation into a separate function

The set of if()-s that determines the preference in cpumask used
for setting things like emulatorpin, vcpupin, etc. is going to be
re-used. Separate it out into a function.

You may think that this changes behaviour, but
qemuProcessPrepareDomainNUMAPlacement() ensures that
priv->autoCpuset is set for VIR_DOMAIN_CPU_PLACEMENT_MODE_AUTO.

Signed-off-by: Michal Privoznik <mprivozn@redhat.com>
Reviewed-by: Kristina Hanicova <khanicov@redhat.com>
Reviewed-by: Andrea Bolognani <abologna@redhat.com>