cpu-exec: Migrate some generic fns to cpu-exec-common

cpu-exec: Migrate some generic fns to cpu-exec-common

The goal is to split the functions such that cpu-exec is CPU specific
content, while cpus-exec-common.c is generic code only. The function
interface to cpu-exec needs to be virtualised to prepare support for
multi-arch and moving these definitions out saves bloating the QOM
interface. So move these definitions out of cpu-exec to a new module,
cpu-exec-common.

Signed-off-by: Peter Crosthwaite <crosthwaite.peter@gmail.com>
Message-Id: <3cefeb3fbbb33031670951a0e74de2778529da3f.1441614289.git.crosthwaite.peter@gmail.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>