Ignore padding when an iovector is supplied

The iSCSI protocol adds padding to a data packet if the data size is not
a multiple of four.  The iovector provided by QEMU does not include such
padding, and libiscsi then complains that there was a protocol error.
This patch fixes this by reading the padding in a separate "recv"
system call.  These packets anyway do not happen in the data path,
where the packet size is a multiple of 512.

This fixes QEMU's scsi-generic backend, which triggered the problem when
the target sent a 66-byte INQUIRY response.

Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>