KVM: selftests: Fold x86's descriptor tables helpers into vcpu_init_sregs()

Now that the per-VM, on-demand allocation logic in kvm_setup_gdt() and
vcpu_init_descriptor_tables() is gone, fold them into vcpu_init_sregs().

Note, both kvm_setup_gdt() and vcpu_init_descriptor_tables() configured the
GDT, which is why it looks like kvm_setup_gdt() disappears.

Opportunistically delete the pointless zeroing of the IDT limit (it was
being unconditionally overwritten by vcpu_init_descriptor_tables()).

Reviewed-by: Ackerley Tng <ackerleytng@google.com>
Link: https://lore.kernel.org/r/20240314232637.2538648-15-seanjc@google.com
Signed-off-by: Sean Christopherson <seanjc@google.com>