CA-70188: Remove invalid pool patches from the database at xapi startup.

A pool_patch is valid if either:

- It has been applied to at least one host, or
- Its file exists on the master filesystem.

It neither of these is true, we can delete the pool_patch object.

Signed-off-by: John Else <john.else@citrix.com>
Merge pull request #1816 from johnelse/ca-70188-rebased

CA-70188: Remove invalid pool patches from the database at xapi startup.