CLOUDSTACK-6023: Non windows instances are created on XenServer with a vcpu-max above supported xenserver limits

Changed the VCPU max limit to 16 and provided a cluster level configuration parameter for this max limit named xen.vm.vcpu.max
(cherry picked from commit 95e41fdf0da50c165a9317847058fce4efeddbcf)

Signed-off-by: Animesh Chaturvedi <animesh@apache.org>