block_util: Simplify RAW synchronous implementation

Using directly preadv and pwritev, we can simply use a RawFd instead of
a file, and we don't need to use the more complex implementation from
the qcow crate.

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

virtio-devices/src/seccomp_filters.rs

@@ -106,6 +106,8 @@ fn virtio_block_thread_rules() -> Result<Vec<SyscallRuleSet>, Error> {
         allow_syscall(libc::SYS_openat),
         allow_syscall(libc::SYS_prctl),
         allow_syscall(libc::SYS_pread64),
+        allow_syscall(libc::SYS_preadv),
+        allow_syscall(libc::SYS_pwritev),
         allow_syscall(libc::SYS_read),
         allow_syscall(libc::SYS_rt_sigprocmask),
         allow_syscall(libc::SYS_sched_getaffinity),