[host] app: calculate and report the required IVSHMEM size

One of the most common issues reported in the support channels is the
IVSHMEM size being too small. This change adds a calculation to
determine an optimal size and uses the new `os_showMessage` platform
method to display a message box to the user with the error.

host/include/interface/platform.h

@@ -40,5 +40,6 @@ void app_quit();
 // these must be implemented for each OS
 const char * os_getExecutable();
 const char * os_getDataPath();
+void os_showMessage(const char * caption, const char * msg);
 
 bool os_blockScreensaver();