xen-image-minimal: fix aarch64 build for non-qemuboot-enabled machines

The qemuboot device tree generation task in qemuboot-xen-dtb.bbclass
requires the QB_SYSTEM_NAME variable to be set to identify the Qemu
binary to use. Skip the task if it is not set.

Reported-and-tested-by: Jon Mason <jdmason@kudzu.us>
Signed-off-by: Christopher Clark <christopher.clark@starlab.io>
Signed-off-by: Bruce Ashfield <bruce.ashfield@gmail.com>