m25p80: qdev-ify drive property

m25p80: qdev-ify drive property

This allows specifying the property via -drive if=none and creating
the flash device with -device.

Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
Signed-off-by: Cédric Le Goater <clg@kaod.org>
Message-id: 1467138270-32481-6-git-send-email-clg@kaod.org
[clg: added an extra fix for sabrelite_init()
      keeping the test on flash_dev did not seem necessary. ]
Signed-off-by: Cédric Le Goater <clg@kaod.org>
Signed-off-by: Peter Maydell <peter.maydell@linaro.org>