hw/block/pflash_cfi02: Implement erase suspend/resume

hw/block/pflash_cfi02: Implement erase suspend/resume

During a sector erase (but not a chip erase), the embeded erase program
can be suspended. Once suspended, the sectors not selected for erasure
may be read and programmed. Autoselect mode is allowed during erase
suspend mode. Presumably, CFI queries are similarly allowed so this
commit allows them as well.

Since guest firmware can use status bits DQ7, DQ6, DQ3, and DQ2 to
determine the current state of sector erasure, these bits are properly
implemented.

Signed-off-by: Stephen Checkoway <stephen.checkoway@oberlin.edu>
Message-Id: <20190426162624.55977-10-stephen.checkoway@oberlin.edu>
Acked-by: Thomas Huth <thuth@redhat.com>
Acked-by: Philippe Mathieu-Daudé <philmd@redhat.com>
Tested-by: Philippe Mathieu-Daudé <philmd@redhat.com>
[PMD: Rebased]
Signed-off-by: Philippe Mathieu-Daudé <philmd@redhat.com>