fw_cfg: Refactor extra pci roots addition

fw_cfg: Refactor extra pci roots addition

Extract extra pci roots addition from pc machine, which could be used by
other machines.

In order to make uefi get the extra roots, it is necessary to write extra
roots into fw_cfg. And only if the uefi knows there are extra roots,
the config spaces of devices behind the root could be obtained.

Signed-off-by: Jiahui Cen <cenjiahui@huawei.com>
Signed-off-by: Yubo Miao <miaoyubo@huawei.com>
Message-Id: <20201119014841.7298-3-cenjiahui@huawei.com>
Reviewed-by: Michael S. Tsirkin <mst@redhat.com>
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>