Handle nested xtrace states (Quantum edition)

The lib/quantum_* directories include plugin files that also
contains the $XTRACE bits at entry and exit.  The nested XTRACE handling overwrote
the value from lib/quantum.  So...make the nested files use their own
XTRACE variables.

Change-Id: Ib5e643371666b21402eef2ec58bfb1dfb7e1ccc4
Merge "Handle nested xtrace states (Quantum edition)"