Bump userfaultfd to 0.8.1

UFFD support is not compatible with newer kernels as there was a new ioctl option added that the older userfaultfd version does not recognize:
```
[PUT /snapshot/load][400] loadSnapshotBadRequest  &{FaultMessage:Load snapshot error: Failed to restore from snapshot: Failed to load guest memory: Error creating guest memory from uffd: Failed to register memory address range with the userfaultfd object: Unrecognized ioctl flags: 284}
```

This was reported in bytecodealliance/userfaultfd-rs#61 and fixed in bytecodealliance/userfaultfd-rs#62

Signed-off-by: Vadim Berezniker <vadim@buildbuddy.io>

src/vmm/Cargo.toml

@@ -25,7 +25,7 @@ serde_json = "1.0.78"
 timerfd = "1.5.0"
 thiserror = "1.0.32"
 displaydoc = "0.2.4"
-userfaultfd = "0.7.0"
+userfaultfd = "0.8.1"
 vhost = { version = "0.10.0", features = ["vhost-user-frontend"] }
 vm-allocator = "0.1.0"
 vm-superio = "0.7.0"