[libvirt] [libvirt-php] Allow block devices in libvirt_domain_disk_add

[Martijn Otto]

I have written a patch to the libvirt_domain_disk_add function which
does the following things:

     1. Allow adding block devices as disks with libvirt_domain_disk_add
     2. Removed the xflags parameter (not relevant as no xml is returned
        anyways)
     3. Added the cache option to control cache mode (either default,
        none, writeback or writethrough)
     4. Added the io option to control the used io mode (either default,
        native or threads)
        
Besides this, I have rewritten it so it now uses the
virDomainAttachDevice function, which is meant for these things, instead
of pasting the device XML by hand into the domain XML.

I have uploaded this to my github clone at
https://github.com/martijnotto/libvirt-php

It would be nice if this could be included in a future release of
lbvirt-php. If the code does not meet a coding standard for libvirt-php
or if there is some other concern, please let me know and I will do what
I can to improve it.

With Regards,
Martijn Otto
Copernica B.V.