qemu-io: Fix recent UI updates

Commit 770e0e0e [*] tried to add 'writev -f', but didn't tweak
the getopt() call to actually let it work.  Likewise, commit
c2e001cc missed implementing 'aio_write -u -z'.  The latter commit
also introduced a leak of ctx.

[*] does it sound "ech0e" in here? :)

Signed-off-by: Eric Blake <eblake@redhat.com>
Message-id: 1463416983-28318-2-git-send-email-eblake@redhat.com
Signed-off-by: Max Reitz <mreitz@redhat.com>