src: Introduce virCgroupFreezerState enum

src: Introduce virCgroupFreezerState enum

So far, only vircgroupv1 implements freezer controller related
callbacks and both work with strings ("THAWED", "FROZEN",
"FREEZING"). This works well with v1 but with CGroupsV2 there are
just two states and they are represented by a number.

Therefore, introduce an enum and implement enum <-> string
conversion for each backend separately.

Signed-off-by: Michal Privoznik <mprivozn@redhat.com>
Reviewed-by: Pavel Hrdina <pavel@hrdina.info>