You should first copy or move the newly built kernel module (xpcie.ko) to a standard kernel module directory, such as:
You'll need to create that directory first. After that, then you must run the depmod command:
Then try loading the module, as root, using just the basename of the module, without the ".ko" extension. "insmod" is still supported but "modprobe" is the currently preferred command to use to install a kernel module, either should work.