Improve VFIO device enumeration performance

[michael-adler]

Description

• Use results of earlier device walks instead of reopening devices.
• Add a lightweight check for busy devices.

[coveralls]

Pull Request Test Coverage Report for Build 7633334986

• 0 of 0 changed or added relevant lines in 0 files are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage decreased (-0.004%) to 64.519%

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Totals
Change from base Build 7632342319:	-0.004%
Covered Lines:	15802
Relevant Lines:	24492

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💛 - Coveralls

[michael-adler]

@anandaravuri and @pcolberg --

This dramatically improves performance of fpgaEnumerate(), especially on systems with a lot of ports. I tested on a machine with about 20 ports and an AFU with 2 children. Opening the AFU dropped from 12 to 2 seconds.

There are two changes you should check:

1. Check whether a port is busy by just opening the vfio group instead of going through the whole container setup and initialization.
2. Discover tokens for a device only once. Once walked, future references to the device use the old token list.

I'm confident about 1. Less so about 2.