KVM: selftests: Init x86's segments during VM creation

Initialize x86's various segments in the GDT during creation of relevant
VMs instead of waiting until vCPUs come along.  Re-installing the segments
for every vCPU is both wasteful and confusing, as is installing KERNEL_DS
multiple times; NOT installing KERNEL_DS for GS is icing on the cake.

Reviewed-by: Ackerley Tng <ackerleytng@google.com>
Link: https://lore.kernel.org/r/20240314232637.2538648-18-seanjc@google.com
Signed-off-by: Sean Christopherson <seanjc@google.com>