nbd: move socket wrappers to qemu-nbd

qemu-nbd is one of the few valid users of qerror_report_err.  Move
the error-reporting socket wrappers there.

Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
Reviewed-by: Fam Zheng <famz@redhat.com>
Signed-off-by: Kevin Wolf <kwolf@redhat.com>
