target/i386/kvm: Free xsave_buf when destroying vCPU

target/i386/kvm: Free xsave_buf when destroying vCPU

Fix vCPU hot-unplug related leak reported by Valgrind:

  ==132362== 4,096 bytes in 1 blocks are definitely lost in loss record 8,440 of 8,549
  ==132362==    at 0x4C3B15F: memalign (vg_replace_malloc.c:1265)
  ==132362==    by 0x4C3B288: posix_memalign (vg_replace_malloc.c:1429)
  ==132362==    by 0xB41195: qemu_try_memalign (memalign.c:53)
  ==132362==    by 0xB41204: qemu_memalign (memalign.c:73)
  ==132362==    by 0x7131CB: kvm_init_xsave (kvm.c:1601)
  ==132362==    by 0x7148ED: kvm_arch_init_vcpu (kvm.c:2031)
  ==132362==    by 0x91D224: kvm_init_vcpu (kvm-all.c:516)
  ==132362==    by 0x9242C9: kvm_vcpu_thread_fn (kvm-accel-ops.c:40)
  ==132362==    by 0xB2EB26: qemu_thread_start (qemu-thread-posix.c:556)
  ==132362==    by 0x7EB2159: start_thread (in /usr/lib64/libpthread-2.28.so)
  ==132362==    by 0x9D45DD2: clone (in /usr/lib64/libc-2.28.so)

Reported-by: Mark Kanda <mark.kanda@oracle.com>
Signed-off-by: Philippe Mathieu-Daudé <f4bug@amsat.org>
Tested-by: Mark Kanda <mark.kanda@oracle.com>
Message-Id: <20220322120522.26200-1-philippe.mathieu.daude@gmail.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>