vl: exit if maxcpus is negative

Signed-off-by: Eduardo Habkost <ehabkost@redhat.com>

---Steps to Reproduce---

When passed a negative number to 'maxcpus' parameter, Qemu aborts
with a core dump.

Run the following command with maxcpus argument as negative number

ppc64-softmmu/qemu-system-ppc64 --nographic -vga none -machine
pseries,accel=kvm,kvm-type=HV -m size=200g -device virtio-blk-pci,
drive=rootdisk -drive file=/home/images/pegas-1.0-ppc64le.qcow2,
if=none,cache=none,id=rootdisk,format=qcow2 -monitor telnet
:127.0.0.1:1234,server,nowait -net nic,model=virtio -net
user -redir tcp:2000::22 -device nec-usb-xhci -smp 8,cores=1,
threads=1,maxcpus=-12

(process:12149): GLib-ERROR **: gmem.c:130: failed to allocate
 18446744073709550568 bytes

Trace/breakpoint trap

Reported-by: R.Nageswara Sastry <rnsastry@linux.vnet.ibm.com>
Signed-off-by: Seeteena Thoufeek <s1seetee@linux.vnet.ibm.com>
Message-Id: <1504511031-26834-1-git-send-email-s1seetee@linux.vnet.ibm.com>
Reviewed-by: Philippe Mathieu-Daudé <f4bug@amsat.org>
