libxl: Create per-domain log file

libxl: Create per-domain log file

Currently, only one log file is created by the libxl driver, with
all output from libxl for all domains going to this one file.

Create a per-domain log file based on domain name, making sifting
through the logs a bit easier.  This required deferring libxl_ctx
allocation until starting the domain, which is fine since the
ctx is not used when the domain is inactive.

    Tested-by: Dario Faggioli <dario.faggioli@citrix.com>