vhost: set/reset device flags internally

vhost: set/reset device flags internally

It does not make sense to ask the application to set/unset the flag
VIRTIO_DEV_RUNNING (that used internal only) at new_device()/
destroy_device() callback.

Instead, it should be set after new_device() succeeds and reset before
destroy_device() is invoked inside vhost lib. This patch fixes it.

Signed-off-by: Yuanhan Liu <yuanhan.liu@linux.intel.com>
Tested-by: Rich Lane <rich.lane@bigswitch.com>
Acked-by: Rich Lane <rich.lane@bigswitch.com>