libvhost-user: Fix resource leak

libvhost-user: Fix resource leak

Free the mmaped memory when we need to mmap new memory
space on vu_set_mem_table_exec() and vu_set_log_base_exec() to
avoid memory leak.

Also close the corresponding fd after mmap() on
vu_set_log_base_exec() to avoid fd leak.

Signed-off-by: Yongji Xie <xieyongji@baidu.com>
Reviewed-by: Michael S. Tsirkin <mst@redhat.com>
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>
Reviewed-by: Marc-André Lureau <marcandre.lureau@redhat.com>