CA-257740: Fix HA broken for NFS

Commit 8a19f82dcf7d6e1737908f7e2380bdb216404991 had introduced
a regression causing attach_from_config to fail, meaning the host
would not boot properly with HA enabled since the attach static VDI
would fail and ha_start_daemon would fail as well because there would
not be any static VDI attached.

Before 8a19f82dcf7d6e1737908f7e2380bdb216404991 FileVDI.load was failing
during attach_from_config since the SR was not mounted but managed to set

   'self.path = raw_path'

before failing.

After 8a19f82dcf7d6e1737908f7e2380bdb216404991 FileVDI.load was still
failing during attach_from_config since the SR was not mounted but
it was leaving

    'self.path = None'

This commit is fixing the problem by checking if 'self.path is None'
and calling self._find_path_with_retries() again to set the path after
the SR is mounted.

Signed-off-by: Stefano Panella <stefano.panella@citrix.com>
Signed-off-by: Mark Syms <mark.syms@citrix.com>
Signed-off-by: Edvin Torok <edvin.torok@citrix.com>
Reviewed-by: Edwin Török <edvin.torok@citrix.com>

drivers/FileSR.py

@@ -607,6 +607,8 @@ def delete(self, sr_uuid, vdi_uuid, data_only = False):
         return super(FileVDI, self).delete(sr_uuid, vdi_uuid, data_only)
 
     def attach(self, sr_uuid, vdi_uuid):
+        if self.path is None:
+            self._find_path_with_retries(vdi_uuid)
         if not self._checkpath(self.path):
             raise xs_errors.XenError('VDIUnavailable', \
                   opterr='VDI %s unavailable %s' % (vdi_uuid, self.path))