[YCONFIG][QEMU][FIX] convert QB-MEM to string
QB_SMP can't be of type int because the qemuboot bbclass expects
it to be the whole -smp QEMU argument.
E.g.: -smp 4 (4 cores)
Convert it to string and improve help string.
QB_SMP can't be of type int because the qemuboot bbclass expects
it to be the whole -smp QEMU argument.
E.g.: -smp 4 (4 cores)
Convert it to string and improve help string.