io: add QIOChannelBuffer class

io: add QIOChannelBuffer class

Add a QIOChannel subclass that is capable of performing I/O
to/from a memory buffer. This implementation does not attempt
to support concurrent readers & writers. It is designed for
serialized access where by a single thread at a time may write
data, seek and then read data back out.

Signed-off-by: Daniel P. Berrange <[email protected]>