Allow unmapping existing mapped ranges in the guest

Previously, the page table manipulation interfaces exported for the
use of guests did not permit unmapping an existing mapped range. This
commit improves that situation, by adding an "unmapped" mapping type,
which code can use to request that a remap operation make a region
not-present.

Signed-off-by: Lucy Menon <168595099+syntactically@users.noreply.github.com>

Author: syntactically

src/hyperlight_common/src/arch/amd64/vmem.rs

@@ -456,6 +456,7 @@ unsafe fn map_page<
                 0 | // R/W - Cow page is never writable
                 PAGE_PRESENT // P   - this entry is present
         }
+        MappingKind::Unmapped => 0,
     };
     unsafe {
         write_entry_updating(op, r.update_parent, r.entry_ptr, pte);

src/hyperlight_common/src/vmem.rs

@@ -196,6 +196,7 @@ pub struct CowMapping {
 
 #[derive(Debug, PartialEq, Clone, Copy)]
 pub enum MappingKind {
+    Unmapped,
     Basic(BasicMapping),
     Cow(CowMapping),
     /* TODO: What useful things other than basic mappings actually

src/hyperlight_host/src/mem/mgr.rs

@@ -65,6 +65,7 @@ fn mapping_kind_to_flags(kind: &MappingKind) -> (MemoryRegionFlags, MemoryRegion
             }
             (flags, MemoryRegionType::Scratch)
         }
+        MappingKind::Unmapped => (MemoryRegionFlags::empty(), MemoryRegionType::Snapshot),
     }
 }
 

src/hyperlight_host/src/sandbox/snapshot.rs

@@ -468,6 +468,7 @@ impl Snapshot {
                             writable: false,
                             executable: bm.executable,
                         }),
+                        MappingKind::Unmapped => continue,
                     };
                     let new_gpa = phys_seen.entry(mapping.phys_base).or_insert_with(|| {
                         let new_offset = snapshot_memory.len();