vl.c: Fix error messages when parsing maxmem parameters

Produce more human readable error messages and fix few spelling
mistakes.

Also remove a redundant check for the max memory size.

Signed-off-by: Peter Krempa <pkrempa@redhat.com>
Acked-by: Michael S. Tsirkin <mst@redhat.com>
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>
Signed-off-by: Peter Krempa <pkrempa@redhat.com>


