fileUtils: Remove support for direct io on memory filesystems

In commits e8f3848d12 and 127936c994, we added support for memory based
filesystems such as tempfs and ramfs. This allows direct io tests to
work when using temporary files in /tmp. These patches disabled direct
io when reading or writing to such filesystems.

Turns out that the code for checking file system type, required for
these changes, has O(N^2) complexity, and profiles we did in the field
on RHEV-H systems with many storage domains show that this code is major
performance issue. We suspect that this issue is related to several
other bugs and customer cases.

Further, code using direct io, such as DirectFile, shouldn't try to
outsmart the caller, disabling direct io magically.  The test writer
should use the appropriate filesystem for such tests.

This patch removes the support for memory filesystems, needed only for
some tests. To prevent failures of the these tests, we use now /var/tmp
instead of /tmp.

I tested disabling memory filesystems on two setups.  The first was a
single customer RHEV-H machine running 3.3 (test performed by customer),
and the second 2 Fedora 19 hosts running master. Both systems are
connected to 30 storage domains, are idle and have no vms running. On
the RHEV-H machine, after disabling memory filesystems, average cpu
usage dropped from 240% to 17%.  On the Fedora systems, average cpu
usage dropped from 10% to 5% when running as spm, and from 5% to 3% when
running as hsm.

Change-Id: I7b652b5c78ff8b48a39df2dc57c02496bec666a1
Bug-Url: https://bugzilla.redhat.com/1095900
Relates-to: https://bugzilla.redhat.com/1074097
Relates-to: https://bugzilla.redhat.com/1081962
Relates-to: http://gerrit.ovirt.org/9595
Relates-to: http://gerrit.ovirt.org/9661
Reviewed-on: http://gerrit.ovirt.org/27441
Reviewed-by: Dan Kenigsberg <danken@redhat.com>
Signed-off-by: Nir Soffer <nsoffer@redhat.com>
Reviewed-on: http://gerrit.ovirt.org/27535
Reviewed-by: Allon Mureinik <amureini@redhat.com>