virtio-pci: use generic logic for command access

virtio-pci: use generic logic for command access

In practice, guests don't generate config requests
that cross a word boundary, so the logic to
detect command word access is correct because
PCI_COMMAND is 0x4. But depending on this is
tricky, further, it will break with guests
that do try to generate a misaligned access
as we pass it to devices without splitting.

Better to use the generic range_covers_byte for this.

Signed-off-by: Michael S. Tsirkin <mst@redhat.com>