FPGAs have become essential infrastructural components as well as publicly rentable resources in cloud and datacenters. Although one tenant is equipped with a couple of individual physical FPGA devices to solve a single problem, cloud FPGAs are still underutilized in many scenarios. Considering the economic cost of such heterogeneous computing resources, it is essential to explore opportunities for FPGA virtualization such that multiple tenants can share one physical device. Unlike the conventional host-centric virtualization approaches considering FPGAs as I/O peripherals, we propose XUNI, an FPGA-centric and self-contained virtual machine (VM) abstraction without involving the host-side virtualization techniques. Specifically, we design a hardware-software co-designed hypervisor for resource management and provisioning of various FPGA VMs. First, XUNI partitions the FPGA fabric into a series of reconfigurable regions that can be flexibly assembled for the deployment of large-scale designs. Second, we introduce both static partition and dynamic allocation schemes for FPGA-side DRAM sharing in XUNI. Last but not least, a hierarchical multi-tenant hardware network stack is built to provide an I/O interface for each FPGA VM. We implement XUNI and conduct infrastructural evaluations on a custom cloud FPGA node populated with an AMD/Xilinx Zynq MPSoC chip. Preliminary results demonstrate that XUNI is capable of handling hundreds of thousands of FPGA-VM-initiated memory requests per second. The hardware network stack exhibits line rate (~100Gbps) when receiving packets with the default 1500-byte MTU size. Moreover, each FPGA VM boots up within hundreds of milliseconds, which is comparable to emerging lightweight host-side VMs.