doc: fix paths of jinja templates for libvirt Xen configuration
doc: fix paths of jinja templates for libvirt Xen configuration

The jinja templates are located in either /etc/qubes/templates or
/usr/share/qubes/templates. /usr/share/templates is not used and
/usr/share/qubes/template/ does not exist.

Fix the paths that are documented in
https://dev.qubes-os.org/projects/core-admin/en/latest/libvirt.html

marmarek/qubes-core-admin,QubesOS/qubes-core-admin,marmarek/qubes-core-admin,marmarek/qubes-core-admin,QubesOS/qubes-core-admin,QubesOS/qubes-core-admin