Increase the size of REMOTE_MIGRATE_COOKIE_MAX to REMOTE_STRING_MAX

During guest migration, if the domain xml is bigger than 16384 which is
easily possible for a guest with good number of disks, message encode fails
for xdr_remote_domain_migrate_perform3_ret().
So, Increase the COOKIE_MAX to STRING_MAX value.
Signed-off-by: NShivaprasad G Bhat <shivaprasadbhat@gmail.com>

src/remote/remote_protocol.x

@@ -86,7 +86,7 @@ const REMOTE_VCPUINFO_MAX = 16384;
 const REMOTE_CPUMAPS_MAX = 8388608;
 
 /* Upper limit on migrate cookie. */
-const REMOTE_MIGRATE_COOKIE_MAX = 16384;
+const REMOTE_MIGRATE_COOKIE_MAX = 4194304;
 
 /* Upper limit on lists of networks. */
 const REMOTE_NETWORK_LIST_MAX = 16384;