exec: invert return value of cpu_physical_memory_get_clean, rename

exec: invert return value of cpu_physical_memory_get_clean, rename

While it is obvious that cpu_physical_memory_get_dirty returns true even if
a single page is dirty, the same is not true for cpu_physical_memory_get_clean;
one would expect that it returns true only if all the pages are clean, but
it actually looks for even one clean page.  (By contrast, the caller of that
function, cpu_physical_memory_range_includes_clean, has a good name).

To clarify, rename the function to cpu_physical_memory_all_dirty and return
true if _all_ the pages are dirty.  This is the opposite of the previous
meaning, because "all are 1" is the same as "not (any is 0)", so we have to
modify cpu_physical_memory_range_includes_clean as well.

Reviewed-by: Fam Zheng <famz@redhat.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>