KVM: selftests: Precisely mask off dynamic fields in CPUID test

KVM: selftests: Precisely mask off dynamic fields in CPUID test

When comparing vCPU CPUID entries against KVM's supported CPUID, mask off
only the dynamic fields/bits instead of skipping the entire entry.
Precisely masking bits isn't meaningfully more difficult than skipping
entire entries, and will be necessary to maintain test coverage when a
future commit enables OSXSAVE by default, i.e. makes one bit in all of
CPUID.0x1 dynamic.

Reviewed-by: Vitaly Kuznetsov <vkuznets@redhat.com>
Link: https://lore.kernel.org/r/20241003234337.273364-3-seanjc@google.com
Signed-off-by: Sean Christopherson <seanjc@google.com>