pvshim: introduce a PV shim defconfig

pvshim: introduce a PV shim defconfig

In order to build a tailored pvshim-only binary from Xen. Switch the
PV shim build from the tools firmware into using the new defconfig.

A diff of the .config generated for the pvshim firmware build before
and after this change shows no differences.

Signed-off-by: Roger Pau Monné <roger.pau@citrix.com>
Acked-by: Ian Jackson <ian.jackson@eu.citrix.com>
Acked-by: Wei Liu <wei.liu2@citrix.com>
Acked-by: Jan Beulich <jbeulich@suse.com>