KVM: selftests: Use TAP-friendly ksft_exit_skip() in __TEST_REQUIRE

Use the TAP-friendly ksft_exit_skip() instead of KVM's custom print_skip()
when skipping a test via __TEST_REQUIRE.  KVM's "skipping test" has no
known benefit, whereas some setups rely on TAP output.

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