vl.c: Use parse_uint_full() for NUMA nodeid

This should catch many kinds of errors that the current code wasn't
checking for:

 - Values that can't be parsed as a number
 - Negative values
 - Overflow
 - Empty string

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