pflash_cfi01: Do not exit() on guest aborting "write to buffer"

pflash_cfi01: Do not exit() on guest aborting "write to buffer"

When a guest tries to abort "write to buffer" (command 0xE8), we print
"PFLASH: Possible BUG - Write block confirm", then exit(1).  Letting
the guest terminate QEMU is not a good idea.  Instead, LOG_UNIMP we
screwed up, then reset the device.

Macro PFLASH_BUG() is now unused; delete it.

Suggested-by: Philippe Mathieu-Daudé <[email protected]>
Signed-off-by: Markus Armbruster <[email protected]>
Reviewed-by: Philippe Mathieu-Daudé <[email protected]>
Reviewed-by: Alex Bennée <[email protected]>
Message-Id: <20190308094610[email protected]>