qemu: Introduce virQEMUCapsFormatHostCPUModelInfo
The CPU model info formating code in virQEMUCapsFormatCache will get
more complicated soon. Separating the code in
virQEMUCapsFormatHostCPUModelInfo will make the result easier to read.
Signed-off-by: Jiri Denemark <jdenemar@redhat.com>