Add config option to configure TB cache size

Qemu>=5.0.0 bumped the default tb-cache size to 1GiB(from 32MiB)
and this made it difficult to run multiple guest VMs on systems
running with lower memory. With Libvirt>=8.0.0 it's possible to
configure lower tb-cache size.

Below config option is introduced to allow configure
TB cache size as per environment needs, this only
applies to 'virt_type=qemu':-

[libvirt]tb_cache_size

Also enable this flag in nova-next job.

[1] https://github.com/qemu/qemu/commit/600e17b26
[2] https://gitlab.com/libvirt/libvirt/-/commit/58bf03f85

Closes-Bug: #1949606
Implements: blueprint libvirt-tb-cache-size
Change-Id: I49d2276ff3d3cc5d560a1bd96f13408e798b256a

.zuul.yaml

@@ -362,6 +362,9 @@
               # machine type attaches more devices by default than pc
               num_pcie_ports: 24
               hw_machine_type: "x86_64=q35"
+              # Use lower TB cache than default(1GiB), only applicable with
+              # libvirt>=8.0.0
+              tb_cache_size: 128
             compute:
               # Switch off the provider association refresh, which should
               # reduce the number of placement calls in steady state. Added in