qtest: allow arbitrarily long sends

qtest currently has a static buffer of size 1024 that if we
overflow, ignores the additional data silently which leads
to hangs or stream failures.

Use glib's string facilities to allow arbitrarily long data,
but split this off into a new function, qtest_sendf.

Static data can still be sent using qtest_send, which avoids
the malloc/copy overhead.

Signed-off-by: John Snow <jsnow@redhat.com>
Reviewed-by: Eric Blake <eblake@redhat.com>
Message-id: 1430864578-22072-2-git-send-email-jsnow@redhat.com
