aarch64: Enable UEFI image loading

Implemented an architecture specific function for loading UEFI binary.

Changed the logic of loading kernel image:
1. First try to load the image as kernel in PE format;
2. If failed, try again to load it as formatless UEFI binary.

Signed-off-by: Jianyong Wu <jianyong.wu@arm.com>

arch/src/aarch64/uefi.rs

@@ -0,0 +1,43 @@
+// Copyright 2020 Arm Limited (or its affiliates). All rights reserved.
+
+use std::io::{Read, Seek, SeekFrom};
+use std::result;
+use vm_memory::{Bytes, GuestAddress, GuestMemory};
+
+/// Errors thrown while loading UEFI binary
+#[derive(Debug)]
+pub enum Error {
+    /// Unable to seek to UEFI image start.
+    SeekUefiStart,
+    /// Unable to seek to UEFI image end.
+    SeekUefiEnd,
+    /// UEFI image too big.
+    UefiTooBig,
+    /// Unable to read UEFI image
+    ReadUefiImage,
+}
+type Result<T> = result::Result<T, Error>;
+
+pub fn load_uefi<F, M: GuestMemory>(
+    guest_mem: &M,
+    guest_addr: GuestAddress,
+    uefi_image: &mut F,
+) -> Result<()>
+where
+    F: Read + Seek,
+{
+    let uefi_size = uefi_image
+        .seek(SeekFrom::End(0))
+        .map_err(|_| Error::SeekUefiEnd)? as usize;
+
+    // edk2 image on virtual platform is smaller than 3M
+    if uefi_size > 0x300000 {
+        return Err(Error::UefiTooBig);
+    }
+    uefi_image
+        .seek(SeekFrom::Start(0))
+        .map_err(|_| Error::SeekUefiStart)?;
+    guest_mem
+        .read_exact_from(guest_addr, uefi_image, uefi_size)
+        .map_err(|_| Error::ReadUefiImage)
+}