vfio: selftests: Split libvfio.h into separate header files

vfio: selftests: Split libvfio.h into separate header files

Split out the contents of libvfio.h into separate header files, but keep
libvfio.h as the top-level include that all tests can use.

Put all new header files into a libvfio/ subdirectory to avoid future
name conflicts in include paths when libvfio is used by other selftests
like KVM.

No functional change intended.

Reviewed-by: Alex Mastro <amastro@fb.com>
Tested-by: Alex Mastro <amastro@fb.com>
Reviewed-by: Raghavendra Rao Ananta <rananta@google.com>
Signed-off-by: David Matlack <dmatlack@google.com>
Link: https://lore.kernel.org/r/20251126231733.3302983-17-dmatlack@google.com
Signed-off-by: Alex Williamson <alex@shazbot.org>