Review 25159

Robert Cowham
Change to make instance dir a directory under /p4 (so on root volume)

This avoids potential problems where /hxdepots is shared on multiple instances.
It also improves performance often.
Left the option to put /p4/common on a shared volume if required.
#3: Change 25190 committed into //guest/perforce_software/sdp/dev/Server 3 0 0
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