jabber: Workaround bug in QtXML: Fix xmlToString when QDomElement contains...

jabber: Workaround bug in QtXML: Fix xmlToString when QDomElement contains Unicode characters above 0xFFFF

Upstream:
https://github.com/psi-im/iris/commit/8612bc340421087cf0ebfd426661ff22f7351270

See also discussion:
https://github.com/psi-im/iris/pull/44
https://github.com/psi-im/iris/pull/43
https://github.com/psi-im/iris/issues/42
https://github.com/psi-im/iris/issues/13
https://bugreports.qt.io/browse/QTBUG-25291

BUG: 225747
BUG: 314272
FIXED-IN: 16.08.1