tests.jumbo: Modify cfg of jumbo tests.

For windows guest when using e1000 the max icmp size is not
"jumbopacket - 28", but "jumbopacket - 42", so modify the cfg.

This patch also modify the coding style to up to the pep8.

Signed-off-by: Yunping Zheng <yunzheng@redhat.com>
tests.jumbo: make jumbo test support macvtap

This patch make jumbo test support macvtap. when using macvtap nic,
before chang the mtu of the macvtap, must change the base interface
mtu.

Signed-off-by: Yunping Zheng <yunzheng@redhat.com>