tools/libxl: improve logging on domain create failure.

tools/libxl: improve logging on domain create failure.

A user reported[0] that xl create failed with just:
    libxl: error: libxl_create.c:892:initiate_domain_create: Unable to set domain build info defaults
and some resulting fallout, but without indicating why it was unable
to set the defaults, even in verbose mode[1].

Go through libxl__domain_{create,build}_info_setdefault and ensure
that each error path logs something.

In most cases this involved simply adding a call to LOG.

In two cases this involved switching from strdup to
libxl__strdup(NOGC) and removing the existing error handling.

When switching from qemu-xen to qemu-xen-traditional (because the
former is not available) log at level INFO rather than VERBOSE, so
the message would normally be printed. Also tweak the language here.

I'm not sure all these messages are reachable (some might be shadowed
by previous error paths) but it seems better to err on the side of
caution.

[0] http://lists.xen.org/archives/html/xen-users/2016-01/msg00125.html
[1] http://lists.xen.org/archives/html/xen-users/2016-01/msg00129.html

Signed-off-by: Ian Campbell <ian.campbell@citrix.com>
Acked-by: Wei Liu <wei.liu2@citrix.com>
Cc: suse.dev@fea.st