pr-helper: avoid error on PR IN command with zero request size

pr-helper: avoid error on PR IN command with zero request size

After reading a PR IN command with zero request size in prh_read_request,
the resp->result field will be uninitialized and the resp.sz field will
be also uninitialized when returning to prh_co_entry.

If resp->result == GOOD (from a previous successful reply or just luck),
then the assert in prh_write_response might not be triggered and
uninitialized response will be sent.

The fix is to remove the whole handling of sz == 0 in prh_co_entry.
Those errors apply only to PR OUT commands and it's perfectly okay to
catch them later in do_pr_out and multipath_pr_out; the check for
too-short parameters in fact doesn't apply in the easy SG_IO case, as
it can be left to the target firmware even.

The result is that prh_read_request does not fail requests anymore and
prh_co_entry becomes simpler.

Reported-by: Dima Stepanov <dimastep@yandex-team.ru>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>