deskutils/virt-manager: add USE_PYTHON=noflavors

[00:01:34] Warning: (deskutils/virt-manager@py35): Error: Duplicated origin for virt-manager-2.0.0: deskutils/virt-manager@py35 AND deskutils/virt-manager. Rerun with -v to see which ports are depending on these.
[00:01:34] Warning: (deskutils/virt-manager@py37): Error: Duplicated origin for virt-manager-2.0.0: deskutils/virt-manager@py37 AND deskutils/virt-manager. Rerun with -v to see which ports are depending on these.

Approved by: portmgr (antoine)