Fixing docstring of hv_xen_unittest.py
Fixing docstring of hv_xen_unittest.py

The introductory comment of hv_xen_unittest.py referred to
the wrong hypervisor.

Signed-off-by: Helga Velroyen <2d0126b1ec6f9ec5cbec4a7012aa14faeaef10f5@google.com>
Reviewed-by: Thomas Thrainer <98bdeac947eb8ca18146e9ce7f092788ec242cd1@google.com>

andir/ganeti,onponomarev/ganeti,ganeti/ganeti,yiannist/ganeti,ganeti-github-testing/ganeti-test-1,leshchevds/ganeti,apyrgio/ganeti,yiannist/ganeti,dimara/ganeti,ganeti/ganeti,mbakke/ganeti,apyrgio/snf-ganeti,leshchevds/ganeti,apyrgio/ganeti,dimara/ganeti,yiannist/ganeti,ganeti-github-testing/ganeti-test-1,andir/ganeti,ganeti/ganeti,onponomarev/ganeti,bitemyapp/ganeti,leshchevds/ganeti,andir/ganeti,grnet/snf-ganeti,apyrgio/snf-ganeti,mbakke/ganeti,grnet/snf-ganeti,mbakke/ganeti,bitemyapp/ganeti