Use correct inferior in Inferior.read_memory et al

Use correct inferior in Inferior.read_memory et al

A user noticed that Inferior.read_memory and a few other Python APIs
will always use the currently selected inferior, not the one passed to
the call.

This patch fixes the bug by arranging to switch to the inferior.  I
found this same issue in several APIs, so this fixes them all.

I also added a few missing calls to INFPY_REQUIRE_VALID to these
methods.

Bug: https://sourceware.org/bugzilla/show_bug.cgi?id=30615
Approved-By: Pedro Alves <pedro@palves.net>