webadmin: Disk hotplug validation - Patch 2 of 2

This change prevents the hot (un)plugging of disks in VMs that do not
support it. The parameter used to determine if hotplugging is supported
was changed from a field hard-coded to a property in the osinfo file.

Change-Id: Ibaf9f42976ea5fc0c7fbc79e3ebe4f9386c94a71
Signed-off-by: Gustavo Pedrosa <gustavo.pedrosa@eldorado.org.br>