configure: set $PYTHON to a full path

This will make it possible to replace it in a shebang line.
Signed-off-by: NPaolo Bonzini <pbonzini@redhat.com>

configure

@@ -908,7 +908,7 @@ for binary in "${PYTHON-python3}" python python2
 do
     if has "$binary"
     then
-        python="$binary"
+        python=$(command -v "$binary")
         break
     fi
 done