tests: add gcov support

Add support for compiling for GCOV test coverage, enabled
with '--enable-gcov' during configure.

Test coverage will be reported after each test.

Signed-off-by: Blue Swirl <blauwirbel@gmail.com>
virtio-pci: replace byte swap hack

Remove byte swaps by declaring the config space
as native endian.

Signed-off-by: Blue Swirl <blauwirbel@gmail.com>