vfio: selftests: Introduce a sysfs lib

Introduce a sysfs library to handle the common reads/writes to the
PCI sysfs files, for example, getting the total number of VFs supported
by the device via /sys/bus/pci/devices/$BDF/sriov_totalvfs. The library
will be used in the upcoming test patch to configure the VFs for a given
PF device.

Since readlink() is quite commonly used in the lib, introduce and use
readlink_safe() to take care of potential buffer overrun errors and to
safely terminate the buffer with '\0'.

Opportunistically, move vfio_pci_get_group_from_dev() to this library as
it falls under the same bucket. Rename it to sysfs_iommu_group_get() to
align with other function names.

Signed-off-by: Raghavendra Rao Ananta <rananta@google.com>
Reviewed-by: Vipin Sharma <vipinsh@google.com>
Reviewed-by: David Matlack <dmatlack@google.com>
Tested-by: David Matlack <dmatlack@google.com>
Link: https://lore.kernel.org/r/20260505212838.1698034-4-rananta@google.com
Signed-off-by: Alex Williamson <alex@shazbot.org>

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