Reduce logging of host hypervisor capabilities to DEBUG level

While the results of invoking libvirt's getCapabilities() API call get
deliberately logged at INFO level[0], doing the same for
getDomainCapabilities() is one bridge too far, because we call it once
per (architecture, default_machine_type) tuple.

So heed the suggestions of Stephen and Eric[1] and reduce it to DEBUG level.

[0] See Ica0dccbfbff89829a7e631adaaf2cde13c449773 at
    https://review.opendev.org/#/c/158334/
[1] https://review.opendev.org/#/c/655268/3/nova/virt/libvirt/host.py@777

Change-Id: Idb4c4380c32f78d639f41cb1b5204578e1a19cd3

Author: Adam Spiers

nova/virt/libvirt/host.py

@@ -775,8 +775,8 @@ def _get_domain_capabilities(self, emulator_bin, arch, machine_type,
             virt_type,
             flags
         )
-        LOG.info("Libvirt host hypervisor capabilities for arch=%s and "
-                 "machine_type=%s:\n%s", arch, machine_type, xmlstr)
+        LOG.debug("Libvirt host hypervisor capabilities for arch=%s and "
+                  "machine_type=%s:\n%s", arch, machine_type, xmlstr)
         caps = vconfig.LibvirtConfigDomainCaps()
         caps.parse_str(xmlstr)
         return caps