Fix OVIRT_CACHE_DIR path issue when AUTOBUILD_SOURCE_ROOT is not set in autobuild

When AUTOBUILD_SOURCE_ROOT is not set,  OVIRT_CACHE_DIR will get a error path.

Change-Id: Icd47b9db5ad832489a07ff8d649d0a55d903a912
Signed-off-by: Wenyi Gao <wenyi@linux.vnet.ibm.com>