Change in vdsm[master]: virt: devices: start the from_xml_tree method

[Code Review]

From Dan Kenigsberg :

Dan Kenigsberg has submitted this change and it was merged.

Change subject: virt: devices: start the from_xml_tree method
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virt: devices: start the from_xml_tree method

Let's start adding to devices the capability to be initialized
from their XML representation.
The end goal is to make possible to properly initialize a device
from either the usual parameters dictionary or a valid XML snippet.

This is only about device construction, no changes about the
state management whatsoever.

Change-Id: I21e80625f1b86cd5a49571d2e121ee479535d8d3
Signed-off-by: Francesco Romani 
---
M lib/vdsm/virt/vmdevices/core.py
M tests/virttests/devicexml_test.py
2 files changed, 104 insertions(+), 0 deletions(-)

Approvals:
  Jenkins CI: Passed CI tests
  Dan Kenigsberg: Looks good to me, approved
  Francesco Romani: Verified
  Martin Polednik: Looks good to me, but someone else must approve