Fix output of uninitialized strings

Commit ffad4116b96e29e0fbe892806f97c0a6c903d30d removed the "scratch buffer"
from check_params, but didn't care for the error messages which actually
included this string to tell the user which option was wrong. Now this string
is uninitialized, so this patch removes it from the message.

This means that the user is only told the whole parameter string and has to
pick the wrong option by himself as the callers of check_params can't know this
value any more. An alternative approach would be to revert that commit and do
whatever is needed to fix the original problem without changing check_params.

Signed-off-by: Kevin Wolf <kwolf@redhat.com>
Signed-off-by: Anthony Liguori <aliguori@us.ibm.com>
