libxl: remove console 0 backend directory from xenstore on devices destroy

libxl: remove console 0 backend directory from xenstore on devices destroy

The first PV console device has an unusual frontend path for
historical reasons (/local/domain/<domid>/console rather than
/local/domain/<domid>/device/console/0).

Therefore we need to check this additional frontend path as well as
those under /local/domain/<domid>/device when tearing down all
backends or else we miss the backend for the console.

As part of this we need to ensure that the frontend directory has a
valid link to the backend, currently this link does not exist.

Fix this by adding libxl__device_frontend_path to return the correct
frontend path, accounting for the special case, and use it from
libxl__device_add_generic. Also add libxl__device_backend_path for
consistency.

This also allows console 0 setup to follow essentially the same code
path as other consoles within libxl_device_console_add, reducing the
special casing required there.

This also fixes the link from backend to frontend which until now gave
a dangling link to the normal device path instead of the exceptional
one.

Signed-off-by: Ian Campbell <ian.campbell@citrix.com>
Signed-off-by: Stefano Stabellini <stefano.stabellini@eu.citrix.com>
committer: Stefano Stabellini <stefano.stabellini@eu.citrix.com>