vmw_pvscsi: Introduce 'x-disable-pcie' backword compatability property

Following the previous patch which changed pvscsi to be a pci express
device, this patch introduces a boolean property 'x-disable-pcie'.

Its default value is false, exposing pvscsi as a pcie device.

Setting 'x-disable-pcie' to 'on' preserves the old 'pci device' (non
express) behavior. This allows migration to older versions.

Signed-off-by: Shmulik Ladkani <shmulik.ladkani@ravellosystems.com>
Message-Id: <1449994112-7054-7-git-send-email-shmulik.ladkani@ravellosystems.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
