mips/cps: Use start-powered-off CPUState property

mips/cps: Use start-powered-off CPUState property

Instead of setting CPUState::halted to 1 in main_cpu_reset(), use the
start-powered-off property which makes cpu_common_reset() initialize it
to 1 in common code.

Also change creation of CPU object from cpu_create() to object_new() and
qdev_realize_and_unref() because cpu_create() realizes the CPU and it's not
possible to set a property after the object is realized.

Signed-off-by: Thiago Jung Bauermann <bauerman@linux.ibm.com>
Message-Id: <20200826055535.951207-6-bauerman@linux.ibm.com>
Reviewed-by: Philippe Mathieu-Daudé <philmd@redhat.com>
Signed-off-by: David Gibson <david@gibson.dropbear.id.au>