Flush the driver name if we change the type of disk

Fix an issue when changing a type of the disk ( related to driver name
for xen which is not always the same contrary to kvm guests).
Add missing Clocks to ClockClocks (avoid void clock CPButton for HVM xen guests)

HVM Xen use variable for clock time.
Fix a bug when Machine is not well refreshed when changing guest type
If the boot device is not defined from libvirt take the first in the list

For XEN pvm, there is no boot device so the buttonBoot was empty,
confusing...
Merge branch 'master' into some-xen-related-fixes
Removing timezone from Clock as it is mis interpreted

Lake of documentation for implementation.
Merge branch 'pr-661'