hypervisor: kvm: Add GUEST_MEMFD and KVM_SET_USER_MEMORY_REGION2 support

Add support for guest_memfd (available in Linux kernel v6.8+), which
enables private memory for confidential VMs.

Key changes:
- Introduce UserMemoryRegion abstraction with guest_memfd fields
- Add From impls between kvm_userspace_memory_region2 and UserMemoryRegion
- Convert all KVM memory region operations from kvm_userspace_memory_region
  to kvm_userspace_memory_region2, with automatic fallback to v1 when
  guest_memfd is not supported
- Add set_user_memory_region() wrapper that dispatches to v1/v2 based on
  kvm_guest_memfd_supported capability
- Create guest_memfd via KVM_CREATE_GUEST_MEMFD ioctl when supported
- Extend KvmDirtyLogSlot to preserve region2 fields across dirty log
  start/stop cycles

This is prerequisite infrastructure for KVM-based confidential computing
that requires private guest memory backed by guest_memfd.

Co-authored-by: Alex Orozco <aorozco@google.com>
Signed-off-by: Keith Adler <kadler@cloudflare.com>
Signed-off-by: Ruben Hakobyan <hruben@meta.com>

hypervisor/src/lib.rs

@@ -118,6 +118,26 @@ pub fn vec_with_array_field<T: Default, F>(count: usize) -> Vec<T> {
     vec_with_size_in_bytes(vec_size_bytes)
 }
 
+/// User memory region structure
+#[derive(Debug, Default, Eq, PartialEq)]
+pub struct UserMemoryRegion {
+    pub slot: u32,
+    pub guest_phys_addr: u64,
+    pub memory_size: u64,
+    pub userspace_addr: u64,
+    pub flags: u32,
+    pub guest_memfd: Option<u32>,
+    pub guest_memfd_offset: Option<u64>,
+}
+
+/// Flags for user memory region
+pub const USER_MEMORY_REGION_READ: u32 = 1;
+pub const USER_MEMORY_REGION_WRITE: u32 = 1 << 1;
+pub const USER_MEMORY_REGION_EXECUTE: u32 = 1 << 2;
+pub const USER_MEMORY_REGION_LOG_DIRTY: u32 = 1 << 3;
+pub const USER_MEMORY_REGION_ADJUSTABLE: u32 = 1 << 4;
+pub const USER_MEMORY_REGION_GUEST_MEMFD: u32 = 1 << 5;
+
 #[derive(Debug)]
 pub enum MpState {
     #[cfg(feature = "kvm")]