qemu: caps: Format SEV platform data into qemuCaps cache

Since we're not saving the platform-specific data into a cache, we're
not going to populate the structure, which in turn will cause a crash
upon calling virNodeGetSEVInfo because of a NULL pointer dereference.
Ultimately, we should start caching this data along with host-specific
capabilities like NUMA and SELinux stuff into a separate cache, but for
the time being, this is a semi-proper fix for a potential crash.

Backtrace (requires libvirtd restart to load qemu caps from cache):
#0 qemuGetSEVInfoToParams
#1 qemuNodeGetSEVInfo
#2 virNodeGetSEVInfo
#3 remoteDispatchNodeGetSevInfo
#4 remoteDispatchNodeGetSevInfoHelper
#5 virNetServerProgramDispatchCall
#6 virNetServerProgramDispatch
#7 virNetServerProcessMsg
#8 virNetServerHandleJob
#9 virThreadPoolWorker
#10 virThreadHelper

https: //bugzilla.redhat.com/show_bug.cgi?id=1612009
Signed-off-by: Erik Skultety <eskultet@redhat.com>
Acked-by: Peter Krempa <pkrempa@redhat.com>
Tested-by: Brijesh Singh <brijesh.singh@amd.com>