CP-4321: Host edition upgrade rules

Free and XenDesktop licenses are left as is.
Advanced, Enterprise and Platinum licenses are converted to per-socket
license.
Anything else is made into a free license.

Signed-off-by: John Else <john.else@citrix.com>
CP-4325: Split License_check.check_expiry up into two functions.

Signed-off-by: John Else <john.else@citrix.com>
Add Test_common.make_pool

Signed-off-by: John Else <john.else@citrix.com>
CP-4325: Skeleton commit for pool.license_state field

Signed-off-by: John Else <john.else@citrix.com>