hw/block/pflash_cfi01: Start state machine as READY to accept commands

hw/block/pflash_cfi01: Start state machine as READY to accept commands

When the state machine is ready to accept command, the bit 7 of
the status register (SR) is set to 1.
The guest polls the status register and check this bit before
writting command to the internal 'Write State Machine' (WSM).

Set SR.7 bit to 1 when the device is created.

There is no migration impact by this change.

Reference: Read Array Flowchart
  "Common Flash Interface (CFI) and Command Sets"
   (Intel Application Note 646)
   Appendix B "Basic Command Set"

Reviewed-by: John Snow <jsnow@redhat.com>
Reviewed-by: Alistair Francis <alistair.francis@wdc.com>
Regression-tested-by: Laszlo Ersek <lersek@redhat.com>
Message-Id: <20190715121338.20600-5-philmd@redhat.com>
Signed-off-by: Philippe Mathieu-Daudé <philmd@redhat.com>