VDI.snapshot: check that SM has this capability
VDI.snapshot: check that SM has this capability

It seems that this capability is reported by SM for most SR types.

Signed-off-by: Gabor Igloi <7535f5501fd71876c93d896691e178fd3a11cfbb@citrix.com>

srowe/xen-api,srowe/xen-api,euanh/xen-api,srowe/xen-api,srowe/xen-api,euanh/xen-api,euanh/xen-api,euanh/xen-api,srowe/xen-api,euanh/xen-api,euanh/xen-api