KVM: selftests: Fix off-by-one initialization of GDT limit

Fix an off-by-one bug in the initialization of the GDT limit, which as
defined in the SDM is inclusive, not exclusive.

Note, vcpu_init_descriptor_tables() gets the limit correct, it's only
vcpu_setup() that is broken, i.e. only tests that _don't_ invoke
vcpu_init_descriptor_tables() can have problems.  And the fact that KVM
effectively initializes the GDT twice will be cleaned up in the near
future.

Signed-off-by: Ackerley Tng <ackerleytng@google.com>
[sean: rewrite changelog]
Link: https://lore.kernel.org/r/20240314232637.2538648-5-seanjc@google.com


Signed-off-by: Sean Christopherson <seanjc@google.com>