cputest: Add "diff" command to cpu-cpuid.py

cputest: Add "diff" command to cpu-cpuid.py

The new command can be used to generate test data for virCPUUpdateLive.

When "cpu-cpuid.py diff x86-cpuid-Something.json" is run, it reads raw
CPUID data stored in x86-cpuid-Something.xml and CPUID data from QEMU
stored in x86-cpuid-Something.json to produce two more CPUID files:
x86-cpuid-Something-enabled.xml and x86-cpuid-Something-disabled.xml.

- x86-cpuid-Something-enabled.xml will contain CPUID bits present in
    x86-cpuid-Something.json (i.e., enabled by QEMU for the "host" CPU)

- x86-cpuid-Something-disabled.xml will contain all CPUID bits from
    x86-cpuid-Something.xml which are not present in
    x86-cpuid-Something.json (i.e., CPUID bits which the host CPU
    supports, but QEMU does not enable them for the "host" CPU)

Signed-off-by: Jiri Denemark <jdenemar@redhat.com>