qemu: Parse unavailable features for CPU models

qemu: Parse unavailable features for CPU models

query-cpu-definitions QMP command returns a list of unavailable features
which prevent CPU models from being usable on the current host. So far
we only checked whether the list was empty to mark CPU models as
(un)usable. This patch parses all unavailable features for each CPU
model and stores them in virDomainCapsCPUModel as a list of usability
blockers.

Signed-off-by: Jiri Denemark <jdenemar@redhat.com>
Reviewed-by: John Ferlan <jferlan@redhat.com>