hi, when i install vitastor on centos7 (kernel 5.9), I get the following output

```shell
linux# LD_PRELOAD=/usr/lib64/qemu-kvm/block-vitastor.so qemu-img convert -f qcow2 /root/vm-disk-1.qcow2 -p -O raw 'vitastor:etcd_host=192.168.30.102\:2379/v3:pool=1:inode=1:size=2148073472'
qemu-img: symbol lookup error: /usr/lib64/qemu-kvm/block-vitastor.so: undefined symbol: bdrv_has_zero_init_1
```
i try building from source and run the same command,get
```shell
linux# LD_PRELOAD=/usr/lib64/qemu-kvm/block-vitastor.so qemu-img convert -f qcow2 /root/vm-disk-1.qcow2 -p -O raw 'vitastor:etcd_host=192.168.30.102\:2379/v3:pool=1:inode=1:size=2148073472'
qemu-img: symbol lookup error: /usr/lib64/qemu-kvm/block-vitastor.so: undefined symbol: PreallocMode_lookup
```
then rebuild QEMU(3.1.1) with  patch (qemu-3.1-vitastor.patch) ,but get
```shell
linux@qemu3.1.1# ./qemu-img convert -f qcow2 /root/vm-disk-1.qcow2 -p -O raw 'vitastor:etcd_host=192.168.30.102\:2379/v3:pool=1:inode=1:size=2148073472'
qemu-img: Unknown…