KVM: selftests: Update x86's set_sregs_test to match KVM's CPUID enforcement

Rework x86's set sregs test to verify that KVM enforces CPUID vs. CR4
features even if userspace hasn't explicitly set guest CPUID.  KVM used to
allow userspace to set any KVM-supported CR4 value prior to KVM_SET_CPUID2,
and the test verified that behavior.

However, the testcase was written purely to verify KVM's existing behavior,
i.e. was NOT written to match the needs of real world VMMs.

Opportunistically verify that KVM continues to reject unsupported features
after KVM_SET_CPUID2 (using KVM_GET_SUPPORTED_CPUID).

Reviewed-by: Maxim Levitsky <mlevitsk@redhat.com>
Reviewed-by: Binbin Wu <binbin.wu@linux.intel.com>
Reviewed-by: Xiaoyao Li <xiaoyao.li@intel.com>
Link: https://lore.kernel.org/r/20241128013424.4096668-7-seanjc@google.com
Signed-off-by: Sean Christopherson <seanjc@google.com>

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