xenapi: Moving tests under tests/virt/xenapi/

We have both a tests/xenapi/ and a tests/virt/xenapi/ directory which
is confusing. This patch standardizes on tests/virt/xenapi/ which
matches the nova/ directory layout.

Change-Id: I324a4730e2477aa1863b17c7ea404ce2932a681c
Merge "xenapi: Moving tests under tests/virt/xenapi/"