Introduce APIs to execute legacy IO admin commands.

It includes: io_legacy_read/write for both common and the device
configuration, io_legacy_notify_info.

In addition, exposing an API to check whether the legacy IO commands are
supported. (i.e. virtio_pci_admin_has_legacy_io()).

Those APIs will be used by the next patches from this series.

Note:
Unlike modern drivers which support hardware virtio devices, legacy
drivers assume software-based devices: e.g. they don't use proper memory
barriers on ARM, use big endian on PPC, etc. X86 drivers are mostly ok
though, more or less by chance. For now, only support legacy IO on X86.
Acked-by: Michael S. Tsirkin <mst@redhat.com>
Signed-off-by: Yishai Hadas <yishaih@nvidia.com>
Link: https://lore.kernel.org/r/20231219093247.170936-7-yishaih@nvidia.comSigned-off-by: Alex Williamson <alex.williamson@redhat.com>