exec: Factor out core logic of check_watchpoint()

exec: Factor out core logic of check_watchpoint()

We want to perform the same checks in probe_write() to trigger a cpu
exit before doing any modifications. We'll have to pass a PC.

Signed-off-by: David Hildenbrand <david@redhat.com>
Reviewed-by: Richard Henderson <richard.henderson@linaro.org>
Message-Id: <20190823100741.9621-9-david@redhat.com>
[rth: Use vaddr for len, like other watchpoint functions;
Move user-only stub to static inline.]
Signed-off-by: Richard Henderson <richard.henderson@linaro.org>