vhost: fix retrieval of numa node in driver

vhost: fix retrieval of numa node in driver

After some testing, it was found that retrieving numa information
about a vhost device via a call to get_mempolicy is more
accurate when performed during the new_device callback versus
the vring_state_changed callback, in particular upon initial boot
of the VM.  Performing this check during new_device is also
potentially more efficient as this callback is only triggered once
during device initialisation, compared with vring_state_changed
which may be called multiple times depending on the number of
queues assigned to the device.

Reorganise the code to perform this check and assign the correct
socket_id to the device during the new_device callback.

Fixes: ee584e9710b9 ("vhost: add driver on top of the library")

Signed-off-by: Ciara Loftus <ciara.loftus@intel.com>
Acked-by: Yuanhan Liu <yuanhan.liu@linux.intel.com>