Fix cache lock for image not consistent

Currently 2 external (file) locks with different name
are created to protect image concurrent access for libvirt driver.
This may lead to image corruption in rare case. This patch change
the lock file name from nova-_var_lib_nova_instances_base_xxxx
to nova-xxxx to make the lock name consistent

Change-Id: I3e60c3bcf456d2392409aa56ba05f4ad8fdc0444
Closes-Bug: #1265452
Merge "Fix cache lock for image not consistent"