Qemu: ask for memory preallocation with large pages

The -mem-prealloc flag should be used when using large pages
This ensures qemu tries to allocate all required memory immediately,
rather than when first used. The latter mode will crash qemu
if hugepages aren't available when accessed, while the former
should gracefully fallback to non-hugepages.

* src/qemu/qemu_conf.c: add -mem-prealloc flag to qemu command line
  when using large pages

src/qemu/qemu_conf.c

@@ -2108,6 +2108,7 @@ int qemudBuildCommandLine(virConnectPtr conn,
                              def->emulator);
             goto error;
         }
+        ADD_ARG_LIT("-mem-prealloc");
         ADD_ARG_LIT("-mem-path");
         ADD_ARG_LIT(driver->hugepage_path);
     }