KVM: selftests: Allocate x86's GDT during VM creation

Allocate the GDT during creation of non-barebones VMs instead of waiting
until the first vCPU is created, as the whole point of non-barebones VMs
is to be able to run vCPUs, i.e. the GDT is going to get allocated no
matter what.

Reviewed-by: Ackerley Tng <ackerleytng@google.com>
Link: https://lore.kernel.org/r/20240314232637.2538648-13-seanjc@google.com


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