stubdoms: generate stubdom config file

stubdoms: generate stubdom config file

This patch removes the need for a second configuration file for
stubdoms: it is going to be automatically generated by the script
stubdom-dm using command line options and xenstore to find any needed
information.
The configuration script will be placed under /etc/xen/stubdoms and
automatically removed when the domain is destroyed.
The only change needed in xend is not to write on xenstore sdl,
opengl and serial command line options for qemu, because stubdoms do
not support them.
It is safe to remove those two options from xenstore because qemu does
not use xenstore to read commans line options.
Finally this patch fixes blkfront disconnections from backends and
display and xauthority variables for pv guests.

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