build-kvmtool.sh: Update the kvm-unit-tests path

The kvm-unit-tests repository is now moved from top directory to
validation/sys-test directory to align with directory structure for
other validation test suites. So update the build script to build
kvm-unit-tests from the correct path.

Signed-off-by: Vivek Gautam <vivek.gautam@arm.com>
Change-Id: I6d1c62ea26e8587538d930a949ec91a4c013eb04