CA-107572: Ignore UUID not found exception during SR scan

VM migration failed because XAPI throw VDI UUID not found exception.
The VDI was already removed, the fix will catch the exception and discard it.

Signed-off-by: Sisimon Soman <sisimon.soman@citrix.com>
Acked-by: Thanos Makatos <thanos.makatos@citrix.com
Imported-by: Siddharth Vinothkumar <siddharth.vinothkumar@citrix.com>

(cherry picked from commit d4100048f6a101e62e3d6d677d2ca7213a2016e0)
CA-122950: Ignore invalid VDI handle exceptions in SR.scan

Signed-off-by: Thanos Makatos <thanos.makatos@citrix.com>
Reviewed-by: Zheng Li <zheng.log@citrix.com>
Imported-by: Siddharth Vinothkumar <siddharth.vinothkumar@citrix.com>

(cherry picked from commit c75ea33750ae71a20da0db753282b831f69cc5ea)
Merge 'CA-122950' and 'CA-107572' into xs64bit

GitHub: closes #127 on xapi-project/sm
CAR-1442: Change NFS mount options

In preparation for read caching, change actimeo=0 to acdirmin=0,acdirmax=0.
Using "actimeo=0" prevents caching file attributes which means that each read
requires a GETATTR call to the NFS server.  This prevents read caching from
being effective.  Changing the mount options to "acdirmin=0,acdirmax=0"
prevents caching directory attributes but allows caching file attributes.  This
allows read caching to be effective.

This reverts the change made to the mount options in CA-63601.  The issue in
that ticket was a transient issue, fixed by a later build, before the change to
the mount options was made.

Signed-off-by: Ross Lagerwall <ross.lagerwall@citrix.com>
Reviewed-by: Thanos Makatos <thanos.makatos@citrix.com>

GitHub: closes #135 on xapi-project/blktap
CAR-1442: Enable no-O_DIRECT read caching for VDIs on EXT and NFS SRs

Pass the "-D" argument to tap-ctl when opening an EXTFileVDI or an NFSFileVDI
to allow read caching by removing O_DIRECT.

Allow disabling read caching per-SR by setting other-config:o_direct to true.

Signed-off-by: Ross Lagerwall <ross.lagerwall@citrix.com>
Reviewed-by: Thanos Makatos <thanos.makatos@citrix.com>

GitHub: closes #135 on xapi-project/blktap
Add unit test related scripts