virtio: pci: Allow exclusion of legacy driver

Add a new config to control whether the driver for legacy virtio PCI
devices is included in the build. VIRTIO_PCI_LEGACY is included by
default when VIRTIO_PCI is selected, but it can also be independently
toggled.

Signed-off-by: Andrew Scull <ascull@google.com>
Reviewed-by: Bin Meng <bmeng.cn@gmail.com>
diff --git a/drivers/virtio/Kconfig b/drivers/virtio/Kconfig
index 863c3fb..586263e 100644
--- a/drivers/virtio/Kconfig
+++ b/drivers/virtio/Kconfig
@@ -37,6 +37,15 @@
 	  This driver provides support for virtio based paravirtual device
 	  drivers over PCI.
 
+config VIRTIO_PCI_LEGACY
+	bool "PCI driver for legacy virtio devices"
+	depends on PCI
+	select VIRTIO
+	default VIRTIO_PCI
+	help
+	  This driver provides support for legacy virtio based paravirtual
+	  device drivers over PCI.
+
 config VIRTIO_SANDBOX
 	bool "Sandbox driver for virtio devices"
 	depends on SANDBOX