Take into consideration races in XtremIOClient3

Take into consideration races in XtremIOClient3

When working with FC and xtremio is using XtremIOClient3 on FC's
terminate_connection we can get VolumeBackendAPIException saying a
resource could not be found when we shouldn't.

The cause is that Client3 is open to race conditions on 2 methods,
find_lunmap and num_of_mapped_volumes, where a list of mappings is first
retrieved and then we iterate this list to retrieved additional info on
the mappings.

The race would happen if one of the mappings is removed from the backend
in the time it takes to retrieve the additional info after we have
retrieved the list.

This patch fixes this issue by ignoring any mappings that have been
removed and are now NotFound when retrieving additional information for
a mapping in those 2 methods.

This patch also fixes this kind of race problems on volume creation
since it uses find_lumap method.

Closes-Bug: #1521143
Change-Id: I40831e04093ff475395870a333211dd0cb60440f