tests/boot-sector: Increase timeout to 600 seconds

tests/boot-sector: Increase timeout to 600 seconds

If QEMU has been compiled with the flags --enable-tcg-interpreter and
--enable-debug, the guest is running incredibly slow. The pxe boot test
can take up to 400 seconds when testing the pseries ppc64 machine. While
we should still look for ways to speed up the test on the pseries machine,
it's better to increase the timeout in this test to 600 seconds anyway to
allow the test to pass successfully now with this unusal configuration
already.

Signed-off-by: Thomas Huth <thuth@redhat.com>
Reviewed-by: Stefan Weil <sw@weilnetz.de>
Signed-off-by: Michael Tokarev <mjt@tls.msk.ru>