qemu: support updating <port isolated='yes|no'/> during device update

qemu: support updating <port isolated='yes|no'/> during device update

This setting can be updating very easily on an already active
interface by just changing it in sysfs. If the bridge used for
connection is also changed, there is no need to separately update it,
because the new setting isf done as a part of connecting to the bridge
anyway.

Signed-off-by: Laine Stump <laine@redhat.com>
Reviewed-by: Ján Tomko <jtomko@redhat.com>