vl.c: validate -numa "cpus" parameter properly

- Accept empty strings without aborting
- Use parse_uint*() to parse numbers
- Abort if anything except '-' or end-of-string is found after the first
  number.
- Check for endvalue < value

Also change the MAX_CPUMASK_BITS warning message from "A max of %d CPUs
are supported in a guest" to "qemu: NUMA: A max of %d VCPUs are
supported".

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