Add configure --with-linux-backend-modules="mod1 mod2"

Add configure --with-linux-backend-modules="mod1 mod2"

This replaces config/Linux.modules with a configure option. As a result
of this change only a single xencomons.in is required, instead of a
xencomons.in.in and sed hackery.

After this change blktap2 and blktap will be loaded at the same time.
This is already done in out-of-tree xencommons scripts, and systemd will
load both modules as well. No harm is expected by loading both modules.

Signed-off-by: Olaf Hering <olaf@aepfle.de>
Acked-by: Ian Campbell <ian.campbell@citrix.com>
Cc: Ian Campbell <ian.campbell@citrix.com>
Cc: Ian Jackson <ian.jackson@eu.citrix.com>
Cc: Stefano Stabellini <stefano.stabellini@eu.citrix.com>
Cc: Wei Liu <wei.liu2@citrix.com>
[ ijc -- ran autogen.sh as requested ]