common: add bt_common_read() function

common: add bt_common_read() function

This function wraps the `read()` function to handle EINTR and partial
reads.

It returns the `count` value on success, or a smaller value than `count`
on error.

Signed-off-by: Francis Deslauriers <francis.deslauriers@efficios.com>
Change-Id: Ib0385855447200d97a58da704dcf68423e857033
Reviewed-on: https://review.lttng.org/c/babeltrace/+/1248
Reviewed-by: Jérémie Galarneau <jeremie.galarneau@efficios.com>