qemu: parse and create -cpu ...,-kvmclock

QEMU supports a bunch of CPUID features that are tied to the kvm CPUID
nodes rather than the processor's.  They are "kvmclock",
"kvm_nopiodelay", "kvm_mmu", "kvm_asyncpf".  These are not known to
libvirt and their CPUID leaf might move if (for example) the Hyper-V
extensions are enabled. Hence their handling would anyway require some
special-casing.

However, among these the most useful is kvmclock; an additional
"property" of this feature is that a <timer> element is a better model
than a CPUID feature.  Although, creating part of the -cpu command-line
from something other than the <cpu> XML element introduces some
ugliness.

Reviewed-by: Jiri Denemark <jdenemar@redhat.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>

tests/qemuargv2xmltest.c

@@ -144,6 +144,10 @@ mymain(void)
     DO_TEST("boot-cdrom");
     DO_TEST("boot-network");
     DO_TEST("boot-floppy");
+    DO_TEST("kvmclock");
+    /* This needs <emulator>./qemu.sh</emulator> which doesn't work here.  */
+    /*DO_TEST("cpu-kvmclock");*/
+
     /* Can't roundtrip xenner arch */
     /*DO_TEST("bootloader");*/
     DO_TEST("clock-utc");