[BUILDER]: Initialize code segment selectors in virtual IDT to 0.

[BUILDER]: Initialize code segment selectors in virtual IDT to 0.
These should not be set until the guest kernel kernel configures an
entry point otherwise the null_trap_bounce() check gets confused.

This change was made to the old domain builder in 12455:3fa6635d04b9
but was lost in the transition to the new builder.

Signed-off-by: Ian Campbell <ian.campbell@xensource.com>