host-validate: Improve CPU flags processing

host-validate: Improve CPU flags processing

Instead of relying on substring search, tokenize the input
and process each CPU flag separately. This ensures CPU flag
detection will continue to work correctly even if we start
looking for CPU flags whose name might appear as part of
other CPU flags' names.

The result of processing is stored in a virBitmap, which
means we don't have to parse /proc/cpuinfo in its entirety
for each single CPU flag we want to check.

Moreover, use of the newly-introduced virHostValidateCPUFlag
enumeration ensures we don't go looking for random CPU flags
which might actually be simple typos.