arch: Check RSDP address does not go past memory

The setup_mptables() call which is not used on ACPI builds has a side
effect of testing whether there was enough RAM which one of the unit
tests was relying on. Add a similar check for the RSDP address.

Signed-off-by: Rob Bradford <robert.bradford@intel.com>

arch/src/lib.rs

@@ -45,6 +45,8 @@ pub enum Error {
     InitramfsAddress,
     /// Error writing module entry to guest memory.
     ModlistSetup(vm_memory::GuestMemoryError),
+    /// RSDP Beyond Guest Memory
+    RSDPPastRamEnd,
 }
 pub type Result<T> = result::Result<T, Error>;