resctrl: Don't assume MBA availability in virResctrlAllocNewFromInfo

resctrl: Don't assume MBA availability in virResctrlAllocNewFromInfo

Weirdly, the existence of /sys/fs/resctrl/info/MB does not always mean
that MBA is available and used on the system.  Instead of assuming that
copy the values from the default (root) allocation.  This also makes it
nicer to use the proper values in case the system does not use
percentages or when the root allocation already limits the bandwidth.

Signed-off-by: Martin Kletzander <mkletzan@redhat.com>
Reviewed-by: Michal Privoznik <mprivozn@redhat.com>