vl.c: Fix off-by-one bug when handling "-numa node" argument

The numa_add() code was unconditionally adding 1 to the get_opt_name()
return value, making it point after the end of the string if no ','
separator is present.

Example of weird behavior caused by the bug:

  $ qemu-img create -f qcow2 this-file-image-has,cpus=5,mem=1000,in-its-name.qcow2 5G
  Formatting 'this-file-image-has,cpus=5,mem=1000,in-its-name.qcow2', fmt=qcow2 size=5368709120 encryption=off cluster_size=65536
  $ ./x86_64-softmmu/qemu-system-x86_64 -S -monitor stdio -numa node 'this-file-image-has,cpus=5,mem=1000,in-its-name.qcow2'
  QEMU 1.3.50 monitor - type 'help' for more information
  (qemu) info numa
  1 nodes
  node 0 cpus: 0
  node 0 size: 1000 MB
  (qemu)

This changes the code to nove the pointer only if ',' is found.

Signed-off-by: Eduardo Habkost <ehabkost@redhat.com>
Reviewed-by: Eric Blake <eblake@redhat.com>
Signed-off-by: Anthony Liguori <aliguori@us.ibm.com>
