block_util, vmm: Add fixed VHD asynchronous implementation

This commit adds the asynchronous support for fixed VHD disk files.

It introduces FixedVhd as a new ImageType, moving the image type
detection to the block_util crate (instead of qcow crate).

It creates a new vhd module in the block_util crate in order to handle
VHD footer, following the VHD specification.

It creates a new fixed_vhd_async module in the block_util crate to
implement the asynchronous version of fixed VHD disk file. It relies on
io_uring.

Signed-off-by: Sebastien Boeuf <sebastien.boeuf@intel.com>

virtio-devices/src/seccomp_filters.rs

@@ -105,6 +105,7 @@ fn virtio_block_thread_rules() -> Result<Vec<SyscallRuleSet>, Error> {
         allow_syscall(libc::SYS_munmap),
         allow_syscall(libc::SYS_openat),
         allow_syscall(libc::SYS_prctl),
+        allow_syscall(libc::SYS_pread64),
         allow_syscall(libc::SYS_read),
         allow_syscall(libc::SYS_rt_sigprocmask),
         allow_syscall(libc::SYS_sched_getaffinity),