libxl: generalise libxl__domain_userdata_lock()

libxl: generalise libxl__domain_userdata_lock()

This function implements a file-based lock with a file name generated
from a domid.

This patch splits it into two, generalising the core of the locking code
into a new libxl__lock_file() function which operates on a specified file,
leaving just the file name generation in libxl__domain_userdata_lock().

This patch also generalises libxl__unlock_domain_userdata() to
libxl__unlock_file() and modifies all call-sites.

Suggested-by: Ian Jackson <ian.jackson@eu.citrix.com>
Signed-off-by: Paul Durrant <pdurrant@amazon.com>
Acked-by: Ian Jackson <ian.jackson@eu.citrix.com>