change all rt_clock references to use millisecond resolution accessors

This was done with:

    sed -i '/get_clock\>.*rt_clock/s/get_clock\>/get_clock_ms/' \
        $(git grep -l 'get_clock\>.*rt_clock' )
    sed -i '/new_timer\>.*rt_clock/s/new_timer\>/new_timer_ms/' \
        $(git grep -l 'new_timer\>.*rt_clock' )

after checking that get_clock and new_timer never occur twice
on the same line.  There were no missed occurrences; however, even
if there had been, they would have been caught by the compiler.

Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
