pc: Add 'etc/boot-cpus' fw_cfg file for machine with more than 255 CPUs

Currently firmware uses 1 byte at 0x5F offset in RTC CMOS
to get number of CPUs present at boot. However 1 byte is
not enough to handle more than 255 CPUs.  So add a new
fw_cfg file that would allow QEMU to tell it.
For compat reasons add file only for machine types that
support more than 255 CPUs.

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