drivers: virtio: Fix writing feature bits, follow VIRTIO spec

This fixes:
 * if-statements that check whether a given bit lies within
specification-defined ranges
 * a behavior in virtio_pci.c -- where it would read feature bits
from the device and write them back whenever the user tried to
set their own -- which led to bugs
 * struct virtio_pci_common_cfg, adding two new fields, bringing
it in line with the specification; the entire struct definition
was reformatted as following the existing style would trigger
warnings

Signed-off-by: Jakub Klimczak <jklimczak@internships.antmicro.com>