[virtmagma] add option to specify GPU device path

This change allows defining DEV_GPU_PATH_OVERRIDE, which overrides the
default /dev/class/gpu directory search path, and instead attempts to
directly create a device using the path specified. This is used when
building for the linux guest, in which the device is found at /dev/wl0.

Test: ran vulkan_unit_tests_linux over guest serial
Change-Id: I20f83c98618bfc4e08632ec29d717b7f3cd64d0e
2 files changed