vmw_pvscsi: Introduce 'x-old-pci-configuration' backword compatability property

Following the previous patches, which introduced various changes in
pvscsi's pci configuration space (device subsystem id and revision, msi
offset), this patch introduces a boolean property
'x-old-pci-configuration' to pvscsi.

Its default value is false, exposing the above changes in the pci config
space.

Setting 'x-old-pci-configuration' to 'on' preserves the old behavior,
which allows migration to older versions.

Signed-off-by: Shmulik Ladkani <shmulik.ladkani@ravellosystems.com>
Message-Id: <1449994112-7054-4-git-send-email-shmulik.ladkani@ravellosystems.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
