scpi_vxi: readability nits in the SCPI via VXI source code

scpi_vxi: readability nits in the SCPI via VXI source code

Address style nits to improve readablity and future maintenance.
Move system header includes before application level includes. Move
assignments out of variable declaration blocks. Separate function calls
and assignments from error checks and flow control. Break a long line.
Add braces around conditional multi line statements.

All these adjustments are cosmetical. Behaviour does not change.