vmm, main: Add optional "hotplug_size" to --mem

This specifies how much address space should be reserved for hotplugging
of RAM. This space is reserved by adding move the start of the device
area by the desired amount.

Signed-off-by: Rob Bradford <robert.bradford@intel.com>

src/main.rs

@@ -94,7 +94,8 @@ fn create_app<'a, 'b>(
                 .long("memory")
                 .help(
                     "Memory parameters \"size=<guest_memory_size>,\
-                     file=<backing_file_path>,mergeable=on|off\"",
+                     file=<backing_file_path>,mergeable=on|off,\
+                     hotplug_size=<hotpluggable_memory_size>\"",
                 )
                 .default_value(&default_memory)
                 .group("vm-config"),
@@ -451,6 +452,7 @@ mod unit_tests {
                     size: 536_870_912,
                     file: None,
                     mergeable: false,
+                    hotplug_size: None,
                 },
                 kernel: None,
                 cmdline: CmdlineConfig {