selftests: KVM: sev_migrate_tests: Fix test_sev_mirror()

Mirrors should not be able to call LAUNCH_START. Remove the call on the
mirror to correct the test before fixing sev_ioctl() to correctly assert
on this failed ioctl.

Cc: Paolo Bonzini <pbonzini@redhat.com>
Cc: Sean Christopherson <seanjc@google.com>
Cc: Marc Orr <marcorr@google.com>
Signed-off-by: Peter Gonda <pgonda@google.com>
Message-Id: <20211208191642.3792819-2-pgonda@google.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>