vhost: wrong value return for VHOST_USER_VRING_GET_BASE

vhost: wrong value return for VHOST_USER_VRING_GET_BASE

When the VM is migrated, the driver sends VHOST_USER_VRING_GET_BASE
message to the device to get the vring offset. The device is
supposed to shut down the vring, and return the current vring offset.
What the code did was to shutdown the vring, initialize the vring,
and return 0 to the driver.

The fix is to first store last_avail_idx in the message and then close
the vring.

Change-Id: I432e9f50f36d89fe53a45e050edcf5e1218caf7a
Signed-off-by: Steven <sluong@cisco.com>