xen/pv: Construct d0v0's GDT properly

xen/pv: Construct d0v0's GDT properly

c/s cf6d39f8199 "x86/PV: properly populate descriptor tables" changed the GDT
to reference zero_page for intermediate frames between the guest and Xen
frames.

Because dom0_construct_pv() doesn't call arch_set_info_guest(), some bits of
initialisation are missed, including the pv_destroy_gdt() which initially
fills the references to zero_page.

In practice, this means there is a window between starting and the first call
to HYPERCALL_set_gdt() were lar/lsl/verr/verw suffer non-architectural
behaviour.

Signed-off-by: Andrew Cooper <andrew.cooper3@citrix.com>
Reviewed-by: Jan Beulich <jbeulich@suse.com>
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This probably wants backporting to Xen 4.7 and later.