should use cache=unsafe for improved performance

[Scott Moser]

https://libvirt.org/formatdomain.html
using 'cache=unsafe' on the disks will probably make installs go much faster.
That renders down to qemu 'cache=unsafe' on the -drive options.
It means that qemu does not sync when the guest vm does a sync() to that disk.

This really improves performance, and for install data you dont really care about the safety of that data.

To my knowledge, qemu will still do a 'close()' on the file descriptors prior to exit, which will get the data ensured to be written to the file.

[Scott Moser (smoser)]

=== modified file 'utah/provisioning/vm.py'
--- utah/provisioning/vm.py 2016-02-04 16:29:18 +0000
+++ utah/provisioning/vm.py 2016-02-04 19:49:27 +0000
@@ -352,6 +352,7 @@
             driver = ElementTree.Element('driver')
             driver.set('name', 'qemu')
             driver.set('type', disk['type'])
+ driver.set('cache', 'unsafe')
             diske.append(driver)
             source = ElementTree.Element('source')
             source.set('file', disk['file'])

I'm testing with the above right now. the install went fine. waiting to make sure it booted and such.