migration: size_t'ify some of qemu-file

migration: size_t'ify some of qemu-file

This is a start on using size_t more in qemu-file and friends;
it fixes up QEMUFilePutBufferFunc and QEMUFileGetBufferFunc
to take size_t lengths and return ssize_t return values (like read(2))
and fixes up all the different implementations of them.

Note that I've not yet followed this deeply into bdrv_ implementations.

Signed-off-by: Dr. David Alan Gilbert <[email protected]>
Message-Id: <1439463094[email protected]>
Reviewed-by: zhanghailiang <[email protected]>
Reviewed-by: Amit Shah <[email protected]>
Signed-off-by: Amit Shah <[email protected]>