vfio: selftests: Move vfio_selftests_*() helpers into libvfio.c

vfio: selftests: Move vfio_selftests_*() helpers into libvfio.c

Move the vfio_selftests_*() helpers into their own file libvfio.c. These
helpers have nothing to do with struct vfio_pci_device, so they don't
make sense in vfio_pci_device.c.

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/20251126231733.3302983-16-dmatlack@google.com
Signed-off-by: Alex Williamson <alex@shazbot.org>