conf: put hostdev PCI backend into a struct

conf: put hostdev PCI backend into a struct

The new struct is virDeviceHostdevPCIDriverInfo, and the "backend"
enum in the hostdevDef will be replaced with a
virDeviceHostdevPCIDriverInfo named "driver'. Since the enum value in
this new struct is called "name", it means that all references to
"backend" will become "driver.name".

This will allow easily adding other items for new attributes in the
<driver> element / C struct, which will be useful once we are using
this new struct in multiple places.

Signed-off-by: Laine Stump <laine@redhat.com>
Reviewed-by: Peter Krempa <pkrempa@redhat.com>