guest: Enable USB input devices for RISC-V virt guests

If USB support is available, we can use USB input devices too.

Reviewed-by: Cole Robinson <crobinso@redhat.com>
Signed-off-by: Andrea Bolognani <abologna@redhat.com>

tests/cli-test-xml/compare/virt-install-riscv64-graphics.xml

@@ -32,6 +32,8 @@
       <source mode="bind"/>
       <target type="virtio" name="org.qemu.guest_agent.0"/>
     </channel>
+    <input type="tablet" bus="usb"/>
+    <input type="keyboard" bus="usb"/>
     <graphics type="vnc" port="-1"/>
     <video>
       <model type="virtio"/>

virtinst/guest.py

@@ -719,7 +719,9 @@ class Guest(XMLBuilder):
         usb_keyboard = False
         if self.os.is_x86() and not self.os.is_xenpv():
             usb_tablet = self.osinfo.supports_usbtablet()
-        if self.os.is_arm_machvirt() or self.os.is_pseries():
+        if (self.os.is_arm_machvirt() or
+            self.os.is_riscv_virt() or
+            self.os.is_pseries()):
             usb_tablet = True
             usb_keyboard = True