I'm looking for a few kernel modules to load i2c-dev and i2c-bcm2708. But the modprobe command returns:

sudo modprobe i2c-dev modprobe: module i2c-dev not found in modules.dep

How can I list all the available modules in the system? In which directory are they located?

Normally modprobe loads modules from sub folders located in /lib/modules/$(uname -r). However all modules should be also listed in the file /lib/modules/$(uname -r)/modules.dep. If some module is present on the system, but is not on the list then run command depmod which will include your module to modules.dep.

Type modprobe and press tab, the autocomplete list should contain all the loadable modules

Use dracut to rebuild initrd with multipath support:

# dracut -f

Add multipathing support:

# dracut --force --add multipath