Slave devices don't need these and the matching code in CloseDevice() has a
IsMaster() condition on freeing these, causing a leak.

==16111== 384 bytes in 4 blocks are definitely lost in loss record 72 of 105
==16111==    at 0x4C28BB4: calloc (vg_replace_malloc.c:467)
==16111==    by 0x42AEE2: AllocDevicePair (devices.c:2707)
==16111==    by 0x4BAA27: AllocXTestDevice (xtest.c:617)
==16111==    by 0x4BA89A: InitXTestDevices (xtest.c:570)
==16111==    by 0x425F5E: InitCoreDevices (devices.c:690)
==16111==    by 0x5ACB2D: main (main.c:257)

Signed-off-by: Peter Hutterer <peter.hutterer@who-t.net>
Reviewed-by: Chase Douglas <chase.douglas@canonical.com>