OvmfPkg/PciHostBridgeLib: Extract GetRootBridges() / FreeRootBridges()

OvmfPkg/PciHostBridgeLib: Extract GetRootBridges() / FreeRootBridges()

Extract PciHostBridgeGetRootBridges() / PciHostBridgeFreeRootBridges() to
PciHostBridgeUtilityLib as common utility functions to share support for
scanning extra root bridges.

No change of functionality.

BZ: https://bugzilla.tianocore.org/show_bug.cgi?id=3059

Cc: Jordan Justen <jordan.l.justen@intel.com>
Cc: Laszlo Ersek <lersek@redhat.com>
Cc: Ard Biesheuvel <ard.biesheuvel@arm.com>
Signed-off-by: Jiahui Cen <cenjiahui@huawei.com>
Signed-off-by: Yubo Miao <miaoyubo@huawei.com>
Message-Id: <20210119011302.10908-8-cenjiahui@huawei.com>
[lersek@redhat.com: keep zeroing of (*Count) centralized]
Reviewed-by: Laszlo Ersek <lersek@redhat.com>