May I ask you what distro are you runnning? To me it seems that you must be using some modern xen kernel on CentOS 6 (4.14.74-6.el6xen.x86_64), is that correct?
make[2]: *** No rule to make target `/root/veeamsnap-master/source/log.o', needed by `/root/veeamsnap-master/source/veeamsnap.o'. Stop.
This might mean that the object file log.o is not there (maybe it hasn't been built for some reason, but the log does not show any info about that). Does the debugging mode show any additional info that might be useful?
Regarding you kernel version: veeamsnap supports only official stable kernels supplied by distros. For CentOS 6 it is 2.6.32-754. Although the module compiled from sources should work fine for the latest kernels on distros that do not ship such kernels by default, it is not officially supported. Since you want to run the agent inside a Xen VM please also keep in mind that the agent currently does not support paravirtualization.