um: UBD Improvements

UBD at present is extremely slow because it handles only
one request at a time in the IO thread and IRQ handler.

The single request at a time is replaced by handling multiple
requests as well as necessary workarounds for short reads/writes.

Resulting performance improvement in disk IO - 30%

Signed-off-by: Anton Ivanov <aivanov@kot-begemot.co.uk>
Signed-off-by: Richard Weinberger <richard@nod.at>