utils: Implement function to pass a buffer to send via a fd to virCommand

utils: Implement function to pass a buffer to send via a fd to virCommand

Implement virCommandSetSendBuffer() that allows the caller to pass a
file descriptor and buffer to virCommand. virCommand will write the
buffer into the file descriptor. That file descriptor could be the
write end of a pipe or one of the file descriptors of a socketpair.
The other file descriptor should be passed to the launched process to
read the data from.

Only implement the function to allocate memory for send buffers
and to free them later on.

Signed-off-by: Stefan Berger <stefanb@linux.ibm.com>
Reviewed-by: Daniel P. Berrangé <berrange@redhat.com>