KVM: selftests: Use a TDP MMU to share EPT page tables between vCPUs

prepare_eptp() currently allocates new EPTs for each vCPU.  memstress has
its own hack to share the EPTs between vCPUs.  Currently, there is no
reason to have separate EPTs for each vCPU, and the complexity is
significant.  The only reason it doesn't matter now is because memstress
is the only user with multiple vCPUs.

Add vm_enable_ept() to allocate EPT page tables for an entire VM, and use
it everywhere to replace prepare_eptp().  Drop 'eptp' and 'eptp_hva' from
'struct vmx_pages' as they serve no purpose (e.g. the EPTP can be built
from the PGD), but keep 'eptp_gpa' so that the MMU structure doesn't need
to be passed in along with vmx_pages.  Dynamically allocate the TDP MMU
structure to avoid a cyclical dependency between kvm_util_arch.h and
kvm_util.h.

Remove the workaround in memstress to copy the EPT root between vCPUs
since that's now the default behavior.

Name the MMU tdp_mmu instead of e.g. nested_mmu or nested.mmu to avoid
recreating the same mess that KVM has with respect to "nested" MMUs, e.g.
does nested refer to the stage-2 page tables created by L1, or the stage-1
page tables created by L2?

Signed-off-by: Yosry Ahmed <yosry.ahmed@linux.dev>
Co-developed-by: Sean Christopherson <seanjc@google.com>
Link: https://patch.msgid.link/20251230230150.4150236-11-seanjc@google.com
Signed-off-by: Sean Christopherson <seanjc@google.com>

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