net: fill in MODULE_DESCRIPTION()s for xen-netback

W=1 builds now warn if module is built without a MODULE_DESCRIPTION().
Add descriptions to the Xen backend network module.

Signed-off-by: Breno Leitao <leitao@debian.org>
Acked-by: Paul Durrant <paul@xen.org>
Link: https://lore.kernel.org/r/20240214152741.670178-2-leitao@debian.org


Signed-off-by: Jakub Kicinski <kuba@kernel.org>