fw_cfg: move FW_CFG_NB_CPUS out of fw_cfg_init1()

PC will use this field in other way, so move it outside the common
code so PC could set a different value, i.e. all CPUs
regardless of where they are coming from (-smp X | -device cpu...).

It's quick and dirty hack as it could be implemented in more generic
way in MashineClass. But do it in simple way since only PC is affected
so far.

Later we can generalize it when another affected target gets support
for -device cpu.

Signed-off-by: Igor Mammedov <imammedo@redhat.com>
Message-Id: <1479212236-183810-3-git-send-email-imammedo@redhat.com>
Reviewed-by: Eduardo Habkost <ehabkost@redhat.com>
Signed-off-by: Eduardo Habkost <ehabkost@redhat.com>