[TOOLS] Improve information displayed by the xm sched-credit command.

[TOOLS] Improve information displayed by the xm sched-credit command.

The improvement is as follows.
 1. The display form is changed like the xm sched-sedf command.
 2. When -d option is omitted, information on all domains is
    displayed.

Examples:
 # xm sched-credit -d vm1
 Name                              ID Weight Cap
 vm1                                1    512  100
 # xm sched-credit
 Name                              ID Weight Cap
 Domain-0                           0    256    0
 vm1                                1    512  100
 vm2                                2    512   50

Signed-off-by: Masaki Kanno <kanno.masaki@jp.fujitsu.com>