xen: make available hvm_fep to non-debug build as well

xen: make available hvm_fep to non-debug build as well

Originally hvm_fep was guarded by NDEBUG, which means it was only
available to debug builds.

However there is value in having it in non-debug builds as well. Users
can use it to run tests in a setup that replicates a production
environment.

Make it clear with a sync_console style warning that this option can't
be used in production setup. Update command line documentation
accordingly. Finally mark Xen as tainted when this feature is used.

Add a kconfig option under x86 to configure hvm_fep.

Signed-off-by: Wei Liu <wei.liu2@citrix.com>
Acked-by: Jan Beulich <jbeulich@suse.com>
Reviewed-by: Andrew Cooper <andrew.cooper3@citrix.com>
Reviewed-by: Doug Goldstein <cardoe@cardoe.com>