Pass dt symbol argument for devices to crosvm

For crosvm to amend device tree for assigned devices, vendors provide
information about vm dtbo node.

Bug: 297313212
Test: vm run-microdroid and see crosvm argument
Change-Id: I52bc532361a40a0db6c1b6b0b25cc3ef29886f48
diff --git a/virtualizationservice/assignable_devices.xsd b/virtualizationservice/assignable_devices.xsd
index 842542e..8f43019 100644
--- a/virtualizationservice/assignable_devices.xsd
+++ b/virtualizationservice/assignable_devices.xsd
@@ -25,6 +25,7 @@
     </xs:element>
     <xs:complexType name="device">
         <xs:attribute name="kind" type="xs:string"/>
+        <xs:attribute name="dtbo_node" type="xs:string"/>
         <xs:attribute name="sysfs_path" type="xs:string"/>
     </xs:complexType>
 </xs:schema>