vhost: fix vhost_log size overflow during migration

vhost: fix vhost_log size overflow during migration

When a guest which doesn't support multiqueue is migrated with a multi queues
vhost-user-blk deivce, a crash will occur like:

0 qemu_memfd_alloc (name=<value optimized out>, size=562949953421312, seals=<value optimized out>, fd=0x7f87171fe8b4, errp=0x7f87171fe8a8) at util/memfd.c:153
1 0x00007f883559d7cf in vhost_log_alloc (size=70368744177664, share=true) at hw/virtio/vhost.c:186
2 0x00007f88355a0758 in vhost_log_get (listener=0x7f8838bd7940, enable=1) at qemu-2-12/hw/virtio/vhost.c:211
3 vhost_dev_log_resize (listener=0x7f8838bd7940, enable=1) at hw/virtio/vhost.c:263
4 vhost_migration_log (listener=0x7f8838bd7940, enable=1) at hw/virtio/vhost.c:787
5 0x00007f88355463d6 in memory_global_dirty_log_start () at memory.c:2503
6 0x00007f8835550577 in ram_init_bitmaps (f=0x7f88384ce600, opaque=0x7f8836024098) at migration/ram.c:2173
7 ram_init_all (f=0x7f88384ce600, opaque=0x7f8836024098) at migration/ram.c:2192
8 ram_save_setup (f=0x7f88384ce600, opaque=0x7f8836024098) at migration/ram.c:2219
9 0x00007f88357a419d in qemu_savevm_state_setup (f=0x7f88384ce600) at migration/savevm.c:1002
10 0x00007f883579fc3e in migration_thread (opaque=0x7f8837530400) at migration/migration.c:2382
11 0x00007f8832447893 in start_thread () from /lib64/libpthread.so.0
12 0x00007f8832178bfd in clone () from /lib64/libc.so.6

This is because vhost_get_log_size() returns a overflowed vhost-log size.
In this function, it uses the uninitialized variable vqs->used_phys and
vqs->used_size to get the vhost-log size.

Signed-off-by: Li Hangjing <lihangjing@baidu.com>
Reviewed-by: Xie Yongji <xieyongji@baidu.com>
Reviewed-by: Chai Wen <chaiwen@baidu.com>
Message-Id: <20190603061524.24076-1-lihangjing@baidu.com>
Cc: qemu-stable@nongnu.org
Reviewed-by: Michael S. Tsirkin <mst@redhat.com>
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>