tests/data/acpi/virt/DSDT.pxb: update with _CCA

Update expected DSDT files accordingly, and re-enable their testing.
diff of disassembly of changed expected files:

    diff -ru -IDisassembly old/tests/data/acpi/virt/DSDT.pxb.dsl new/tests/data/acpi/virt/DSDT.pxb.dsl
    --- old/tests/data/acpi/virt/DSDT.pxb.dsl       2021-02-23 09:54:18.566781350 -0500
    +++ new/tests/data/acpi/virt/DSDT.pxb.dsl       2021-02-23 09:57:51.952816428 -0500
                 Name (_BBN, 0x80)  // _BBN: BIOS Bus Number
                 Name (_UID, 0x80)  // _UID: Unique ID
                 Name (_STR, Unicode ("pxb Device"))  // _STR: Description String
    +            Name (_CCA, One)  // _CCA: Cache Coherency Attribute
                 Name (_PRT, Package (0x80)  // _PRT: PCI Routing Table
                 {
                     Package (0x04)

Signed-off-by: Jiahui Cen <cenjiahui@huawei.com>
Signed-off-by: Xingang Wang <wangxingang5@huawei.com>
Reviewed-by: Michael S. Tsirkin <mst@redhat.com>
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>

tests/qtest/bios-tables-test-allowed-diff.h

 /* List of comma-separated changed AML files to ignore */
-"tests/data/acpi/virt/DSDT.pxb",