virt-host-validate: Allow longer list of CPU flags

virt-host-validate: Allow longer list of CPU flags

On various occasions, virt-host-validate parses /proc/cpuinfo to
learn about CPU flags (see virHostValidateGetCPUFlags()). It does
so, by reading the file line by line until the line with CPU
flags is reached. Then the line is split into individual flags
(using space as a delimiter) and the list of flags is then
iterated over.

This works, except for cases when the line with CPU flags is too
long. Problem is - the line is capped at 1024 bytes and on newer
CPUs (and newer kernels), the line can be significantly longer.
I've seen a line that's ~1200 characters long (with 164 flags
reported).

Switch to unbounded read from the file (getline()).

Resolves: https://issues.redhat.com/browse/RHEL-39969
Signed-off-by: Michal Privoznik <mprivozn@redhat.com>
Reviewed-by: Jiri Denemark <jdenemar@redhat.com>