CA-99034: Fix unit for DS:memory_internal_free.

Imported from xen-api.git 9a51d817f9d850c62932b10e20ae70d3f3481708

Signed-off-by: Siddharth Vinothkumar <siddharth.vinothkumar@citrix.com>
Imported-by: John Else <john.else@citrix.com>