vfio: Add O_CLOEXEC flag to vfio device fd

Add the default O_CLOEXEC flag for device file descriptors.  This is
generally considered a safer option as it allows the user a race free
option to decide whether file descriptors are inherited across exec,
with the default avoiding file descriptor leaks.

Reported-by: Yann Droneaud <ydroneaud@opteya.com>
Signed-off-by: Alex Williamson <alex.williamson@redhat.com>