CLOUDSTACK-6358: As a part of supporting dynamic guest OS defined by user, removing the hard-coded dependencies.
This patch is for XenServer.

1. Local testing on XenServer 6.0.2
2. Successfully got up system VMs
3. Successfully created a CentOS VM
4. Created VM snapshot and reverted to snapshot
5. Manipulated guest_os_hypervisor table for mapping, and checked for correct propagation of platform emulator.

Signed off by :- Nitin Mehta<nitin.mehta@citrix.com>
CLOUDSTACK-6358: As a part of supporting dynamic guest OS defined by user, removing the hard-coded dependencies.
This patch is for KVM

1. Local testing on KVM
2. Successfully got up system VMs
3. Successfully created a CentOS VM
4. Snapshots are not supported for KVM

 Signed off by :- Nitin Mehta<nitin.mehta@citrix.com>