stubdom: fix cdrom changing

stubdom: fix cdrom changing

Hi all,
the current code to change a cdrom doesn't work with stubdoms:

- media_filename set at boot time doesn't have the proper
value (that in the stubdom case is the frontend path and not the
filename);

- when a cdrom watch event is triggered, the code to decide whether the
new cdrom is valid and different from the current cdrom doesn't work for
stubdoms;

both issues are fixed by this patch, in particular now media_filename
consistently holds the frontend path for stubdoms while bs->filename
holds the filename (like in the normal qemu case) to allow comparisons
with the old cdrom filename.

Signed-off-by: Stefano Stabellini <stefano.stabellini@eu.citrix.com>