qemu-io-cmds: Assert that global and nofile commands don't use ct->perms

It would be a bug for a command with the CMD_NOFILE_OK or
CMD_FLAG_GLOBAL flags set to also set the ct->perms field,
because the former says "OK for a file not to be open"
but the latter is a check on a file.

Add an assertion in qemuio_add_command() so we can catch that
sort of buggy command definition immediately rather than it
being a bug that only manifests when a particular set of
command line options is used.

(Coverity gets confused about this (CID 1371723) and reports
that we might dereference a NULL blk pointer in this case,
because it can't tell that that code path never happens with
the cmdinfo_t that we have. This commit won't help unconfuse
it, but it does fix the underlying issue.)

Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
Message-id: 1490967529-4767-1-git-send-email-peter.maydell@linaro.org
Signed-off-by: Max Reitz <mreitz@redhat.com>