pci: Fix offsets in *_pci_config_*

On VMware, pci_readw(PCI_CFG_DEVICE_ID) returns the *vendor ID*.
pci_readw(PCI_CFG_VENDOR_ID) returns vendor ID as well.

Compare to FreeBSD implementation of read/write PCI config space,
FreeBSD masks lower bit of offset when write to PCI_CONFIG_ADDRESS, and
adds lower bit of offset to PCI_CONFIG_DATA.

http://fxr.watson.org/fxr/source/amd64/pci/pci_cfgreg.c#L206

This patch changes accessing method in OSv to the FreeBSD way.  Tested
on QEMU/KVM and VMware.

Reviewed-by: Glauber Costa <glommer@cloudius-systems.com>
Signed-off-by: Takuya ASADA <syuu@cloudius-systems.com>
Signed-off-by: Pekka Enberg <penberg@cloudius-systems.com>