Adds constant for max message size

BUG: 72654144
Change-Id: I69c962da515715824ae7057ed9439f5b26ef444b
diff --git a/common/vsoc/shm/socket_forward_layout.h b/common/vsoc/shm/socket_forward_layout.h
index 970662d..9f4de05 100644
--- a/common/vsoc/shm/socket_forward_layout.h
+++ b/common/vsoc/shm/socket_forward_layout.h
@@ -25,11 +25,13 @@
 namespace layout {
 namespace socket_forward {
 
+constexpr std::size_t kMaxPacketSize = 8192;
+
 struct QueuePair {
   // Traffic originating from host that proceeds towards guest.
-  CircularPacketQueue<16, 8192> host_to_guest;
+  CircularPacketQueue<16, kMaxPacketSize> host_to_guest;
   // Traffic originating from guest that proceeds towards host.
-  CircularPacketQueue<16, 8192> guest_to_host;
+  CircularPacketQueue<16, kMaxPacketSize> guest_to_host;
 
   enum QueueState : std::uint32_t {
     INACTIVE = 0,