block-migration: add lock

Some state is shared between the block migration code and its AIO
callbacks.  Once block migration will run outside the iothread,
the block migration code and the AIO callbacks will be able to
run concurrently.  Protect the critical sections with a separate
lock.  Do the same for completed_sectors, which can be used from
the monitor.

Reviewed-by: Orit Wasserman <owasserm@redhat.com>
Reviewed-by: Juan Quintela <quintela@redhat.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
Signed-off-by: Juan Quintela <quintela@redhat.com>
