iscsi: fix deadlock during login

If the connection is interrupted before the first login is successfully
completed qemu-kvm is waiting forever in qemu_aio_wait().

This is fixed by performing an sync login to the target. If the
connection breaks after the first successful login errors are
handled internally by libiscsi.

Signed-off-by: Peter Lieven <pl@kamp.de>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
