blockdev: acquire AioContext in eject, change, and block_passwd

By acquiring the AioContext we avoid race conditions with the dataplane
thread which may also be accessing the BlockDriverState.

Fix up eject, change, and block_passwd in a single patch because
qmp_eject() and qmp_change_blockdev() both call eject_device().  Also
fix block_passwd while we're tackling a command that takes a block
encryption password.

Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>
Reviewed-by: Max Reitz <mreitz@redhat.com>
Signed-off-by: Max Reitz <mreitz@redhat.com>
Signed-off-by: Kevin Wolf <kwolf@redhat.com>
