colo: claim in find_and_check_chardev

This factors out claiming of chardev, and changes the call to
non-fatal to return an error like the rest of the chardev checks.

Signed-off-by: Marc-André Lureau <marcandre.lureau@redhat.com>
Message-Id: <20161022095318.17775-15-marcandre.lureau@redhat.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
