arm/virt: Fix virt_machine_device_plug_cb() error API violation

arm/virt: Fix virt_machine_device_plug_cb() error API violation

The Error ** argument must be NULL, &error_abort, &error_fatal, or a
pointer to a variable containing NULL.  Passing an argument of the
latter kind twice without clearing it in between is wrong: if the
first call sets an error, it no longer points to NULL for the second
call.

virt_machine_device_plug_cb() passes @errp to
cryptodev_builtin_sym_close_session() in a loop.  Harmless, because
cryptodev_builtin_sym_close_session() can't actually fail.  Fix by
dropping its Error ** parameter.

Cc: Peter Maydell <peter.maydell@linaro.org>
Cc: qemu-arm@nongnu.org
Signed-off-by: Markus Armbruster <armbru@redhat.com>
Reviewed-by: Philippe Mathieu-Daudé <philmd@redhat.com>
Message-Id: <20200422130719.28225-6-armbru@redhat.com>