core: refactor OvfVmReader#readHardwareSection

The method OvfVmReader#readHardwareSection was too long. By extracting
the handling of each hardware type into a separate method and using
the relatively new capability in Java of switch-case statement on
String, this patch simplify that method.

Change-Id: I403489679eb97359842a3662278b7312a81911fd
Signed-off-by: Arik Hadas <ahadas@redhat.com>