vmm: seccomp: Add SYS_fsync to vcpu thread

Fix disk hot unplug failure caused by seccomp SIGSYS kill.

When performing disk hot unplug, the vcpu thread calls fsync()
on the block device file descriptor to flush pending I/O.
The seccomp filter previously blocked SYS_fsync, triggering SIGSYS
and terminating the vcpu thread, which makes the hot unplug
operation fail. This issue exists on both x86 and AArch64.

Strace log snippet captured during failure:
```
[pid 3118852] fsync(142) = 142
[pid 3118852] ---SIGSYS {si_signo=SIGSYS,si_code=SYS_SECCOMP,si_call_addr=0xffff9c931df8, si_syscall=__NR_fsync,si_arch=AUDIT_ARCH_AARCH64}
```
Add unrestricted SYS_fsync entry to vcpu thread syscall allowlist,
consistent with existing file I/O syscalls such as fcntl and fstat.

Signed-off-by: yanjianqing <yanjianqing@kylinos.cn>

vmm/src/seccomp_filters.rs

@@ -902,6 +902,7 @@ fn vcpu_thread_rules(
         ),
         (libc::SYS_fcntl, vec![]),
         (libc::SYS_fstat, vec![]),
+        (libc::SYS_fsync, vec![]),
         (libc::SYS_futex, vec![]),
         (libc::SYS_getcwd, vec![]),
         (libc::SYS_getrandom, vec![]),