vl.c: extend -m option to support options for memory hotplug

vl.c: extend -m option to support options for memory hotplug

Add following parameters:
  "slots" - total number of hotplug memory slots
  "maxmem" - maximum possible memory

"slots" and "maxmem" should go in pair and "maxmem" should be greater
than "mem" for memory hotplug to be enabled.

Signed-off-by: Igor Mammedov <imammedo@redhat.com>
Reviewed-by: Michael S. Tsirkin <mst@redhat.com>
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>
MST: fix build on 32 bit