gitlab: Add issue template for reporting a bug

gitlab: Add issue template for reporting a bug

When reporting an issue in gitlab, the project can define a template for
various scenarios which are meant to guide the users to add the relevant
information the project needs to the reported issue.

Add a template for a bug report against libvirt. The template adds
sections which motivate users to add version information and also link
to documentation about fetching logs and such.

Note that markdown seems to be the only supported format for now.

Signed-off-by: Peter Krempa <pkrempa@redhat.com>
Reviewed-by: Daniel P. Berrangé <berrange@redhat.com>