libxl: Rename ev_devlock to ev_slowlock

libxl: Rename ev_devlock to ev_slowlock

We are going to introduce a different lock based on the same
implementation as the ev_devlock but with a different path. The
different slowlock will be differentiated by calling different _init()
functions.

So we rename libxl__ev_devlock to lib__ev_slowlock, but keep
libxl__ev_devlock_init().

Some log messages produced ev_slowlock are changed to print the
name of the lock file (userdata_userid).

Signed-off-by: Anthony PERARD <anthony.perard@citrix.com>
Acked-by: Ian Jackson <ian.jackson@eu.citrix.com>
Release-acked-by: Juergen Gross <jgross@suse.com>