fix of assigning multiple netdev_extra_params

Currently we added a comma for netdev_extra_params in framework,
if we want to add multiple parameters in more than one times,
we should not use comma prefix in first parameter string.

eg. add multiple parameters more than one times:
  netdev_extra_params = "vhost=on"
  netdev_extra_params += ",sndbuf=512"

This patch moved the comma out of framework, then we can
set the parameter conveniently. We don't care about if it's the
first assignment, just using comma prefix for all params strings.

Signed-off-by: Amos Kong <akong@redhat.com>
virttest.qemu_vm: Raise VMConfigMissingError for 9p cfgs

The original VMCreateError exception arguments are incorrect.
And actually these 2 places should raise a VMConfigMissingError.

Signed-off-by: Qingtang Zhou <qzhou@redhat.com>