Commit 1a1562f5 prepared a VMSTATE_CPU()
macro for device-style VMStateDescription registration, but missed to
adapt cpu_exec_init(), so that the "cpu_common" VMStateDescription was
still registered for AlphaCPU (fe31e737)
and OpenRISCCPU (da697214). Fix this.

Cc: Richard Henderson <rth@twiddle.net>
Tested-by: Jia Liu <proljc@gmail.com>
Signed-off-by: Andreas Färber <afaerber@suse.de>