KVM: selftests: Dedup MSR index list helpers, simplify dedicated test

Consolidate the helper for retrieving the list of save/restore MSRs and
the list of feature MSRs, and use the common helpers in the related
get_msr_index_features test.  Switching to the common helpers eliminates
the testcase that KVM returns the same -E2BIG result if the input number
of MSRs is '1' versus '0', but considered that testcase isn't very
interesting, e.g. '0' and '1' are equally arbitrary, and certainly not
worth the additional code.

Signed-off-by: Sean Christopherson <seanjc@google.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>