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
diff --git a/include/hw/boards.h b/include/hw/boards.h
index 2d2e2be..184d245 100644
--- a/include/hw/boards.h
+++ b/include/hw/boards.h
@@ -113,6 +113,8 @@
     char *firmware;
 
     ram_addr_t ram_size;
+    ram_addr_t maxram_size;
+    uint64_t   ram_slots;
     const char *boot_order;
     char *kernel_filename;
     char *kernel_cmdline;