KVM: selftests: Allocate x86's TSS at VM creation

Allocate x86's per-VM TSS at creation of a non-barebones VM.  Like the
GDT, the TSS is needed to actually run vCPUs, i.e. every non-barebones VM
is all but guaranteed to allocate the TSS sooner or later.

Reviewed-by: Ackerley Tng <ackerleytng@google.com>
Link: https://lore.kernel.org/r/20240314232637.2538648-16-seanjc@google.com
Signed-off-by: Sean Christopherson <seanjc@google.com>