exec.c: check memory backend file size with 'size' option

If the memory backend file is not large enough to hold the required 'size',
Qemu will report error and exit.

Signed-off-by: Haozhong Zhang <haozhong.zhang@intel.com>
Message-Id: <20161027042300.5929-3-haozhong.zhang@intel.com>
Reviewed-by: Eduardo Habkost <ehabkost@redhat.com>
Message-Id: <20161102010551.2723-1-haozhong.zhang@intel.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>

exec.c

@@ -1321,6 +1321,13 @@ static void *file_ram_alloc(RAMBlock *block,
         goto error;
     }
 
+    if (file_size > 0 && file_size < memory) {
+        error_setg(errp, "backing store %s size 0x%" PRIx64
+                   " does not match 'size' option 0x" RAM_ADDR_FMT,
+                   path, file_size, memory);
+        goto error;
+    }
+
     memory = ROUND_UP(memory, block->page_size);
 
     /*