CP-9365: XenCenter work for HVM linux support

Signed-off-by: Gabor Apati-Nagy <gabor.apati-nagy@citrix.com>
CP-9365: XenCenter work for HVM linux support
-changes following the code review

Signed-off-by: Gabor Apati-Nagy <gabor.apati-nagy@citrix.com>
CP-9365: XenCenter work for HVM linux support
-Unsubscribing from guest_metrics changed event in UnregisterEventListeners()
CP-9365: XenCenter work for HVM linux support
-Fixing issue found by the reviewer: null reference exception when the VM restarts
Merge pull request #200 from GaborApatiNagy/hvm2

CP-9365: XenCenter work for HVM linux support