libxenlight: write stubdoms logs to file

libxenlight: write stubdoms logs to file

It turns out that there is a better way to write stubdoms logs to file
than using libxl_console_attach: qemu is the one that provides the
console backend for stubdoms and qemu is able to redirect a serial to
file, so we can use this feature to make sure the first stubdom
console is always redirected to a logfile.

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