tools: libxl: Handle failure to create qemu dm logfile

tools: libxl: Handle failure to create qemu dm logfile

If libxl_create_logfile fails for some reason then
libxl__create_qemu_logfile previously just carried on and dereferenced
the uninitialised logfile.

Check for the error from libxl_create_logfile, which has already
logged for us.

This was reported as Debian bug #784880.

Reported-by: Russell Coker <russell@coker.com.au>
Signed-off-by: Ian Campbell <ian.campbell@citrix.com>
Cc: 784880@bugs.debian.org
Acked-by: Wei Liu <wei.liu2@citrix.com>