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/qemuxml2argvdata/qemuxml2argv-cpu-host-kvmclock.args

@@ -0,0 +1,4 @@
+LC_ALL=C PATH=/bin HOME=/home/test USER=test LOGNAME=test ./qemu.sh -S -M pc \
+-cpu host,-kvmclock -enable-kvm -m 214 -smp 6 \
+-nographic -monitor unix:/tmp/test-monitor,server,nowait -no-acpi -boot n -net \
+none -serial none -parallel none -usb