KVM: selftests: Allocate a dedicated guest page for x86 L2 guest stack

Instead of relying on the L1-provided stack for L2, which is usually an
array on L1's own stack, allocate a dedicated page of VM memory for the
L2 stack in vcpu_alloc_{vmx/svm}() and use that as L2's RSP in the
VMCS/VMCB instead of the L1-provided value.

Most L1 guest code does not do anything with the L2 stack other than
stuff it in RSP, so this change is transparent and the L1-provided stack
is silently ignored. The only exception is memstress nested L1 code
which puts the vCPU index on L2's stack, so update this code to use the
newly allocated stack.

L1-provided stacks will be dropped and cleaned up separately.

Suggested-by: Sean Christopherson <seanjc@google.com>
Signed-off-by: Yosry Ahmed <yosry@kernel.org>
Link: https://patch.msgid.link/20260610003030.2957261-3-seanjc@google.com
Signed-off-by: Sean Christopherson <seanjc@google.com>

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