Changes:
- Adding a new table 'hypervisor_capabilities' that will record capabilities for each hypervisor version. Added db schema changes for this.
- Currently a few capabilities have been added, namely, 'max_guests_limit' and 'security_group_enabled'
- Added a new column 'hypervisor_version' to host table. StartupRouting command now takes in this parameter. It should be set when a host connects.
- If a host's hypervisor version is not present, we find all the capabilities rows for that hypervisor type and use the first record.
- 'max_guests_limit' is the maximum number of running guest Vms that a host can have for the given hypervisor.
- Host Allocators use this limit and skip a host if the number of running VMs on that host exceeds this limit.
