docs/system/devices/usb: Replace the non-existing "qemu" binary

We don't ship a binary that is simply called "qemu", so we should
avoid this in the documentation. Use the configurable binary name
via "|qemu_system|" instead.

Signed-off-by: Thomas Huth <thuth@redhat.com>
Reviewed-by: Michael Tokarev <mjt@tls.msk.ru>
Signed-off-by: Michael Tokarev <mjt@tls.msk.ru>

docs/system/devices/usb.rst

@@ -18,7 +18,7 @@ emulation uses less resources (especially CPU).  So if your guest
 supports XHCI (which should be the case for any operating system
 released around 2010 or later) we recommend using it:
 
-    qemu -device qemu-xhci
+    |qemu_system| -device qemu-xhci
 
 XHCI supports USB 1.1, USB 2.0 and USB 3.0 devices, so this is the
 only controller you need.  With only a single USB controller (and