virhook: support hooks placed in $driver.d/

virhook: support hooks placed in $driver.d/

It is easier for management software (and subsequently
distributions) to install hook script under
/etc/libvirt/hooks/$driver.d/ and have libvirt execute them in
alphabetical order. To maintain backwards compatibility,
/etc/libvirt/hooks/$driver hook script is executed the first
followed by scripts from the $driver.d directory.

The stdio is chained between the scripts. The output of the first
script is input of the second and so on.

Signed-off-by: Dmitry Nesterenko <dmitry.nesterenko@virtuozzo.com>
Signed-off-by: Michal Privoznik <mprivozn@redhat.com>
Reviewed-by: Michal Privoznik <mprivozn@redhat.com>