vhost: introduce safe API for GPA translation

This new rte_vhost_va_from_guest_pa API takes an extra len
parameter, used to specify the size of the range to be mapped.
Effective mapped range is returned via len parameter.

This issue has been assigned CVE-2018-1059.

Reported-by: Yongji Xie <xieyongji@baidu.com>
Signed-off-by: Maxime Coquelin <maxime.coquelin@redhat.com>