hw/arm/virt: fix cmdline parsing bug with CPU options and smp > 1

hw/arm/virt: fix cmdline parsing bug with CPU options and smp > 1

The recently introduced feature that allows 32 bit guests to be
executed under KVM on a 64-bit host incorrectly handles the case
where more than 1 cpu is specified using '-smp N'

For instance, this invocation of qemu

  qemu-system-aarch64 -M virt -cpu cortex-a57,aarch64=off -smp 2

produces the following error

  qemu-system-aarch64: Expected key=value format, found aarch64

which is caused by the destructive parsing performed by
cpu_common_parse_features(), resulting in subsequent attempts
to parse the CPU option string (for each additional CPU) to fail.

So duplicate the string before parsing it, and free it directly
afterwards.

Signed-off-by: Ard Biesheuvel <ard.biesheuvel@linaro.org>
Reviewed-by: Greg Bellows <greg.bellows@linaro.org>
Message-id: 1425402380-10488-1-git-send-email-ard.biesheuvel@linaro.org
Signed-off-by: Peter Maydell <peter.maydell@linaro.org>