libxl: introduce libxl_userdata_unlink

libxl: introduce libxl_userdata_unlink

This will be used in later patch for xl to remove its "xl" userdata
file.

Both CTX lock and userdata lock are taken in this API. CTX lock is taken
to maintain locking hierarchy, but it also has a side effect to protect
against R-M-W by other threads. Userdata lock is used to protect against
domain destruction.

In general application should not rely on these internal locks to
protect its own userdata files. It should deploys its own lock if it
cares.

Signed-off-by: Wei Liu <wei.liu2@citrix.com>
Acked-by: Ian Campbell <ian.campbell@citrix.com>