Allow multiple parameters for schedinfo

Allow multiple parameters for schedinfo

virsh schedinfo was able to set only one parameter at a time (not
counting the deprecated options), but it is useful to set more at
once, so this patch adds the possibility to do stuff like this:

virsh schedinfo <domain> cpu_shares=0 vcpu_period=0 vcpu_quota=0 \
emulator_period=0 emulator_quota=0

Invalid scheduler options are reported as well.  These were previously
reported only if the command hadn't updated any values (when
cmdSchedInfoUpdate returned 0).

Resolves: https://bugzilla.redhat.com/show_bug.cgi?id=810078
Resolves: https://bugzilla.redhat.com/show_bug.cgi?id=919372
Resolves: https://bugzilla.redhat.com/show_bug.cgi?id=919375