target/i386/sev: Mark unreachable code with g_assert_not_reached()

target/i386/sev: Mark unreachable code with g_assert_not_reached()

The unique sev_encrypt_flash() invocation (in pc_system_flash_map)
is protected by the "if (sev_enabled())" check, so is not
reacheable.
Replace the abort() call in sev_es_save_reset_vector() by
g_assert_not_reached() which meaning is clearer.

Reviewed-by: Connor Kuehl <ckuehl@redhat.com>
Reviewed-by: Paolo Bonzini <pbonzini@redhat.com>
Signed-off-by: Philippe Mathieu-Daudé <philmd@redhat.com>
Message-Id: <20211007161716.453984-11-philmd@redhat.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>