Add invariant TSC cpu flag

Add invariant TSC cpu flag

Add suport for invariant TSC flag (CPUID 0x80000007, bit 8 of EDX).
If this flag is enabled, the TSC ticks at a constant rate across
all ACPI P-, C- and T-states.

This can be enabled by adding:
<feature name='invtsc'/>
to the <cpu> element.

Migration and saving the domain does not work with this flag.

QEMU support: http://git.qemu.org/?p=qemu.git;a=commitdiff;h=303752a

The feature name "invtsc" differs from the name "" used by the linux kernel:
https://git.kernel.org/cgit/linux/kernel/git/torvalds/linux.git/tree/arch/x86/kernel/cpu/powerflags.c?id=30321c7b#n18