vmm: Improve resiliency of image type handling

Add an image_type to DiskConfig to specify the image type. If none is
specified autodetect the image type but disable potentially unsafe
behaviour in the QCOW2 backend by disabling the backing file support.

If the image type is autodetected then fix it in the config so that it
will be persistant across reboots and migrations/snapshot & restores.
This also handles the case where the image type was not specified as
part of the disk configuration.

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

block/src/lib.rs

@@ -30,12 +30,13 @@ pub mod vhdx_sync;
 
 use std::alloc::{Layout, alloc_zeroed, dealloc};
 use std::collections::VecDeque;
-use std::fmt::Debug;
+use std::fmt::{self, Debug};
 use std::fs::File;
 use std::io::{self, IoSlice, IoSliceMut, Read, Seek, SeekFrom, Write};
 use std::os::linux::fs::MetadataExt;
 use std::os::unix::io::AsRawFd;
 use std::path::Path;
+use std::str::FromStr;
 use std::time::Instant;
 use std::{cmp, result};
 
@@ -1005,12 +1006,44 @@ pub trait AsyncAdaptor {
     }
 }
 
-#[derive(PartialEq, Eq, Debug)]
+#[derive(Serialize, Deserialize, Clone, Copy, Debug, PartialEq, Eq, Default)]
 pub enum ImageType {
     FixedVhd,
     Qcow2,
     Raw,
     Vhdx,
+    #[default]
+    Unknown,
+}
+
+impl fmt::Display for ImageType {
+    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
+        match self {
+            ImageType::FixedVhd => write!(f, "vhd"),
+            ImageType::Qcow2 => write!(f, "qcow2"),
+            ImageType::Raw => write!(f, "raw"),
+            ImageType::Vhdx => write!(f, "vhdx"),
+            ImageType::Unknown => write!(f, "unknown"),
+        }
+    }
+}
+
+pub enum ImageTypeParseError {
+    InvalidValue(String),
+}
+
+impl FromStr for ImageType {
+    type Err = ImageTypeParseError;
+
+    fn from_str(s: &str) -> Result<Self, Self::Err> {
+        match s.to_lowercase().as_str() {
+            "vhd" => Ok(ImageType::FixedVhd),
+            "qcow2" => Ok(ImageType::Qcow2),
+            "raw" => Ok(ImageType::Raw),
+            "vhdx" => Ok(ImageType::Vhdx),
+            _ => Err(ImageTypeParseError::InvalidValue(s.to_string())),
+        }
+    }
 }
 
 const QCOW_MAGIC: u32 = 0x5146_49fb;