qemu.tests.cfg: adjust the timeout in transfer_file_over_ipv6

Tranfer file over ipv6 needs more time to finish.

Signed-off-by: Xiangmin Han <xhan@redhat.com>
Merge pull request #54 from Xiangmin/fix_transfer_file_over_ipv6_timeout

qemu.tests.cfg: adjust the timeout in transfer_file_over_ipv6