KVM: selftests: Add coverage for the EFER_LMSLE_MBZ defeature

Add coverage for KVM's virtualization of EFER_LMSLE_MBZ,
CPUID.80000008H:EBX[bit 20], which is set when the CPU does *not* support long
mode segment limits.  Verify that KVM's enumeration of the defeature matches
the expectation derived from hardware plus kvm_amd's "nested" module param, so
that the assertion doesn't compare KVM's enumeration against itself, and that
WRMSR(EFER), nested VMRUN, and KVM_SET_SREGS reject EFER.LMSLE=1 if and only
if guest CPUID enumerates the defeature.  Also verify that host-initiated
KVM_SET_MSRS is exempt from the check, as host-initiated writes skip guest
CPUID checks so that userspace can set MSRs before it sets guest CPUID.

Extend svm_nested_efer_test rather than add yet another test binary for a
single CPUID bit; the test already has the L1/L2 harness needed to exercise
EFER.LMSLE.  Note, only a CPU that supports LMSLE, i.e. Rome and earlier,
exercises the emulation path; elsewhere KVM enumerates the defeature straight
from hardware.

Opportunistically drop the unnecessary #include of vmx.h.

Assisted-by: LLM
Signed-off-by: Jim Mattson <jmattson@google.com>
[sean: drop specific APM volume references, use common "nested" getter]
Link: https://patch.msgid.link/20260930212114.3500045-6-seanjc@google.com
Signed-off-by: Sean Christopherson <seanjc@google.com>

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