vm-allocator: Add an allocator KVM memory slots

Add an allocator for KVM memory slots, a finite resource in the kernel
- these need to be manipulated across different crates with a common
dependency on this crate.

Signed-off-by: Rob Bradford <rbradford@rivosinc.com>

vm-allocator/src/lib.rs

@@ -21,3 +21,5 @@ pub use crate::gsi::GsiAllocator;
 #[cfg(target_arch = "x86_64")]
 pub use crate::gsi::GsiApic;
 pub use crate::system::SystemAllocator;
+mod memory_slot;
+pub use memory_slot::MemorySlotAllocator;