Increase MAX_CPUMASK_BITS from 255 to 288

so that it would be possible to increase maxcpus limit
for x86 target. Keep spapr/virt_arm at limit they used
to have 255.

Signed-off-by: Igor Mammedov <imammedo@redhat.com>
Reviewed-by: Andrew Jones <drjones@redhat.com>
Reviewed-by: Eduardo Habkost <ehabkost@redhat.com>
Signed-off-by: Eduardo Habkost <ehabkost@redhat.com>
