Change tests/misc-bsd-callout to C++

Rename tests/misc-bsd-callout.c to tests/misc-bsd-callout.cc - we'll
need it in C++ in the upcoming patch set.

No changes were needed for this code to continue compiling.

Signed-off-by: Nadav Har'El <nyh@cloudius-systems.com>
Signed-off-by: Avi Kivity <avi@cloudius-systems.com>