fix detection strategy for MSVC/VS2017+

* fixes the MSVC/VS 2017 detection problem discussed within <https://github.com/tboox/xmake/issues/225>

.# Discussion

MSVC/VS versions >= 15.0 will no longer be setting registry entries for tool location
purposes. But `vswhere` (included with MSVC/VS versions >= 15.2, and placed in a
guaranteed location) can instead be used to find the needed path.

`os.runv(...)` is used as the tool to gather `vswhere` output instead of `($shell ...)`
in order to avoid some `($shell ...)` limitations. Commands containing white space and/or
special characters (eg, '[', ')') are difficult or impossible to pass through the current
`($shell ...)` implementation, causing application exceptions. Both white space and the
noted special characters are needed to construct the required `vswhere` command.

ref: <https://github.com/Microsoft/vswhere/blob/master/README.md> @@ <https://archive.is/mEmdu>

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