qemu-iotests: Fix _filter_qemu

$QEMU_PROG happens to be 'qemu' in my setup, so this sed command
replaces a bit too much. Restrict it to the start of the line and to
when it's followed by a colon, i.e. the form used by error messages.

Signed-off-by: Kevin Wolf <kwolf@redhat.com>
Reviewed-by: Eric Blake <eblake@redhat.com>
Reviewed-by: Stefan Hajnoczi <stefanha@redhat.com>