libxl: update check-xl-disk-parse

libxl: update check-xl-disk-parse

The block-attach command now returns 1 when fails. Update first test
case to expect return value 1 instead of 255.

The parser now doesn't generate output for default values. Remove them
from expected output.

According to 417e6b70 ("libxl: add option for discard support to xl disk
configuration"), the "discard=" variant is never supported, delete two
test cases with that variant.

Reported-by: Jim Fehlig <jfehlig@suse.com>
Signed-off-by: Wei Liu <wei.liu2@citrix.com>
Tested-by: Jim Fehlig <jfehlig@suse.com>
Acked-by: Ian Campbell <ian.campbell@citrix.com>