net: remove muldiv64()

muldiv64() is used to convert nanoseconds to microseconds.

    x = muldiv64(qemu_clock_get_ns(..), 1000000, get_ticks_per_sec());

As  get_ticks_per_sec() is 10^9, it can be replaced by:

    x = qemu_clock_get_us(..);

Signed-off-by: Laurent Vivier <lvivier@redhat.com>
Reviewed-by: Stefan Hajnoczi <stefanha@redhat.com>