xl errors if /var/log/xen doesn't exist

[Andrew Cooper <andrew.cooper3@citrix.com>]

Hello,

As part of running XenRT on a XenServer experimental build on top of
Xen-4.4.0-rc1, I encountered the following error which is far from helpful:

$ xl create /root/minios.cfg
xl: fatal error: xl_cmdimpl.c:457: No such file or directory: logfile =
open(fullname, O_WRONLY|O_CREAT|O_APPEND, 0644)
Parsing config from /root/minios.cfg
libxl: error: libxl_exec.c:118:libxl_report_child_exitstatus:
daemonizing child [32476] exited with error status 3
Parsing config from /root/minios.cfg

The root cause of the issue is that the directly /var/log/xen didn't
exist (because I messed up the packaging).

Because of the CHK_SYSCALL() macro which prints a string-ized version of
its argument, the error message omits the key detail (i.e. which file
doesn't exist).  Furthermore, the string 'fullname' is a complete path
which has been returned from "libxl_create_logfile()" without error.

If the failure to create a logfile is a fatal error (which itself is
probably not correct - what about a RO dom0 which strictly logs to
rsyslog?), then either xl or libxl should assume the responsibility of
ensuring that its logging directory actually exists, and
"libxl_create_logfile()" seems like a prime candidate to assume this
responsibility.

~Andrew