Added app.report_each_app().

Calls app:report() on each app.
LoadGen: Updated to work the new app API.

Example:

    # ./snabbswitch designs.loadgen.loadgen apps/ipv6/selftest.cap.input 0000:05:00.1 0000:8a:00.1
    Waiting for linkup. ok
    Waiting for linkup. ok
    Transmissions (last 1 sec):
    0000:05:00.1    TXDGPC (TX packets)     5,555,035       GOTCL (TX octets)       566,509,228
    0000:8a:00.1    TXDGPC (TX packets)     5,562,714       GOTCL (TX octets)       567,267,638
    Transmissions (last 1 sec):
    0000:05:00.1    TXDGPC (TX packets)     10,246,367      GOTCL (TX octets)       1,045,086,878
    0000:8a:00.1    TXDGPC (TX packets)     10,246,082      GOTCL (TX octets)       1,045,065,138
    Transmissions (last 1 sec):
    0000:05:00.1    TXDGPC (TX packets)     10,246,077      GOTCL (TX octets)       1,045,063,166
    0000:8a:00.1    TXDGPC (TX packets)     10,246,277      GOTCL (TX octets)       1,045,085,438

Fixes #128.