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