qemu.tests: Update operstate_always_up in set_link

As in newest version qemu-kvm, guest can get link status when
link id is netdev id. So make this configurable to fit both
old and new version of qemu-kvm

Signed-off-by: Yiqiao Pu <ypu@redhat.com>
qemu.tests: Update up and down state for status=off test in set_link

When set up status=off in command line. The status get from guest will
always be unknown.

Signed-off-by: Yiqiao Pu <ypu@redhat.com>
qemu.tests: Use cmd_output_safe in set_link

As set_link use serial login, so it also met the randome message
from kernel problem. Use cmd_output_safe in it.

Also fix some line more than 80 character.

Signed-off-by: Yiqiao Pu <ypu@redhat.com>
Merge pull request #1329 from ypu/set_link

Set link