vhost-user: Add reconnect support

vhost-user: Add reconnect support

QEMU 2.7 introduces reconnect support. However it doesn't send
last state of last_avail_idx in the SET_VRING_BASE.

With this patch we are overriding SET_VRING_BASE with value taken
from used->idx.

It is a bit hackish as it basically overrides SET_VRING_BASE
provided value but it should work in most of the cases.
This is what author of reconnect code is also suggesting in his post:

https://lists.gnu.org/archive/html/qemu-devel/2016-06/msg01495.html

Change-Id: I10e5f401c409d2989bf6c43eaea377800a9c76df
Signed-off-by: Damjan Marion <[email protected]>