libxl: reimplement buffer for bootloading and drop data if buffer is full.

libxl: reimplement buffer for bootloading and drop data if buffer is full.

Implement a buffer for the bootloading process that appends data to
the end until it's full. Drop the whole buffer if a timeout has
occurred and the buffer is full. Prevents the bootloader from getting
stuck when using ptys with small buffers.

Signed-off-by: Roger Pau Monne <roger.pau@entel.upc.edu>
Committed-by: Ian Jackson <ian.jackson@eu.citrix.com>