libxl: Fix broken build attach/detach controller device

libxl: Fix broken build attach/detach controller device

Commit id '44304c6eb' added the API libxlDomainAttachControllerDevice
inside a conditional LIBXL_HAVE_PVUSB, but called that function outside
the conditional in libxlDomainAttachDeviceLive.

Similarly, the API libxlDomainDetachControllerDevice was added inside a
conditional LIBXL_HAVE_PVUSB, but called outside the conditional in
libxlDomainDetachDeviceLive.

This patch adds the conditional LIBXL_HAVE_PVUSB around those two calls
from within the switch.