Default issue template for libvirt.git

[Peter Krempa]

Hi,

we've recently added templates for issues for the libvirt repo [1].

Unfortunately gitlab made a weird design decision, where the default
template is not actually picked from the templates comitted to the
repository, but needs to be added in the repo config in the web-ui into
a separate textarea. This unfortunately creates a gap where the
repo-comitted stuff can actually diverge from the default.

I've asked Pavel to copy the 'bug.md' template into the "default" one so
even if users don't select any they will see the bug one by default.

My reasoning despite being vulnerable to diverging from the in-repo
state is that we've added templates so that we prevent too-little
information being filled. Having to select a template requires
additional steps and also asks for more than the reporter might have
reported normally, so it may be ofputting for them to select it
manually. Thus we rather want to force a default.

Alternatively we can add another comment on top asking them to switch to
a different template for enhancement such as:

<!-- If you are requesting an enhancement, use the appropriate template -->

Peter

[1] https://www.redhat.com/archives/libvir-list/2020-December/msg00003.html