dump: Acquire BQL around vm_start() in dump thread

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>