dump: Acquire BQL around vm_start() in dump thread

This fixes an assertion failure in the following backtrace:

    __GI___assert_fail
    memory_region_transaction_commit
    memory_region_add_eventfd
    virtio_pci_ioeventfd_assign
    virtio_bus_set_host_notifier
    virtio_blk_data_plane_start
    virtio_bus_start_ioeventfd
    virtio_vmstate_change
    vm_state_notify
    vm_prepare_start
    vm_start
    dump_cleanup
    dump_process
    dump_thread
    start_thread
    clone

vm_start need BQL, acquire it if doing cleaning up from main thread.

Signed-off-by: Fam Zheng <famz@redhat.com>
Message-Id: <20170503072819.14462-1-famz@redhat.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>

dump.c

@@ -77,7 +77,13 @@ static int dump_cleanup(DumpState *s)
     memory_mapping_list_free(&s->list);
     close(s->fd);
     if (s->resume) {
+        if (s->detached) {
+            qemu_mutex_lock_iothread();
+        }
         vm_start();
+        if (s->detached) {
+            qemu_mutex_unlock_iothread();
+        }
     }
 
     return 0;
@@ -1804,6 +1810,7 @@ void qmp_dump_guest_memory(bool paging, const char *file,
 
     if (detach_p) {
         /* detached dump */
+        s->detached = true;
         qemu_thread_create(&s->dump_thread, "dump_thread", dump_thread,
                            s, QEMU_THREAD_DETACHED);
     } else {

include/sysemu/dump.h

@@ -157,6 +157,7 @@ typedef struct DumpState {
     uint32_t sh_info;
     bool have_section;
     bool resume;
+    bool detached;
     ssize_t note_size;
     hwaddr memory_offset;
     int fd;