qed: introduce bdrv_qed_init_state

This will be used in the next patch, which will call bdrv_qed_do_open
with a CoMutex taken.  bdrv_qed_init_state provides a nice place to
initialize it.

Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
Message-Id: <20170629132749.997-9-pbonzini@redhat.com>
Reviewed-by: Eric Blake <eblake@redhat.com>
Reviewed-by: Stefan Hajnoczi <stefanha@redhat.com>
Reviewed-by: Fam Zheng <famz@redhat.com>
Signed-off-by: Fam Zheng <famz@redhat.com>
