hw/scsi: Introduce VHOST_SCSI_COMMON symbol in Kconfig

hw/scsi: Introduce VHOST_SCSI_COMMON symbol in Kconfig

Instead of adding 'vhost-scsi-common.c' twice (for VHOST_SCSI and
VHOST_USER_SCSI), have it depend on VHOST_SCSI_COMMON, selected by
both symbols.

Signed-off-by: Philippe Mathieu-Daudé <philmd@linaro.org>
Reviewed-by: Thomas Huth <thuth@redhat.com>
Reviewed-by: Richard Henderson <richard.henderson@linaro.org>
Message-Id: <20230524093744.88442-3-philmd@linaro.org>
Reviewed-by: Michael S. Tsirkin <mst@redhat.com>
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>
Reviewed-by: Alex Bennée <alex.bennee@linaro.org>