vm-allocator: Introduce an MMIO hole address allocator

With this new AddressAllocator as part of the SystemAllocator, the
VMM can now decide with finer granularity where to place memory.

By allocating the RAM and the hole into the MMIO address space, we
ensure that no memory will be allocated by accident where the RAM or
where the hole is.
And by creating the new MMIO hole address space, we create a subset
of the entire MMIO address space where we can place 32 bits BARs for
example.

Signed-off-by: Sebastien Boeuf <sebastien.boeuf@intel.com>

arch/src/lib.rs

@@ -33,6 +33,15 @@ pub enum Error {
 }
 pub type Result<T> = result::Result<T, Error>;
 
+#[derive(PartialEq)]
+pub enum RegionType {
+    /// RAM type
+    Ram,
+    /// Reserved type. Designate a region which should not be considered as
+    /// RAM. Useful to specify a PCI hole for instance.
+    Reserved,
+}
+
 // 1MB.  We don't put anything above here except the kernel itself.
 pub const HIMEM_START: GuestAddress = GuestAddress(0x100000);