block/raw-posix: Build fix for O_ASYNC

block/raw-posix: Build fix for O_ASYNC

Commit eeb6b45d48800e96f67ef2a5c80332557fd45ddb (block: raw-posix image
file reopen) broke the build on OpenIndiana.

illumos has no O_ASYNC. Exclude it from flags to be compared
and instead assert that it is not set where defined.

Cf. e61ab1da7e98357da47c54d8f893b9bd6ff2f7f9 for qemu-ga.

Cc: [email protected] (1.3.x)
Cc: Jeff Cody <[email protected]>
Suggested-by: Paolo Bonzini <[email protected]>
Signed-off-by: Andreas Färber <[email protected]>
Signed-off-by: Stefan Hajnoczi <[email protected]>