virtio_pci_modern_dev: warn once on invalid status

vp_modern_get_status() returns the raw device_status byte as read
from the common configuration structure (struct virtio_pci_common_cfg,
mapped via the VIRTIO_PCI_CAP_COMMON_CFG capability). That byte should
only ever contain some combination of the status bits defined by the
virtio spec (bits 0-3, 6-7); bits 4 and 5 are reserved and a
spec-compliant device must never set them. A value with any other bit
set means either the device is violating the spec, or the read never
reached real device state at all -- e.g. because a write of 0x0000 to
the PCI_COMMAND register (config space offset 4) clears the Memory
Space Enable bit, causing the device to stop responding to
memory-mapped register accesses -- effectively simulating an
unresponsive/removed device without a real hot-unplug. In that case
the MMIO read returns the bus's synthesized all-ones response instead
of real device state.

Add VIRTIO_STATUS_ERROR() to the uapi header to recognize such values,
and warn once from vp_modern_get_status() when it sees one, so the
bogus status is visible at its source rather than only showing up as
confusing behavior in callers.

Signed-off-by: Abhin Parekadan Jose <abhinjoses@gmail.com>
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>
Message-ID: <20260802174059.4082-2-abhinjoses@gmail.com>

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