virsh-domain: Refactor error paths for cmdCPUStats

virsh-domain: Refactor error paths for cmdCPUStats

This patch fixes the following issues in the cpu-stats virsh command:

1) Renames label failed_params to no_memory to match coding style
2) Uses proper typed parameter cleanup in error paths to avoid leaks
3) Adds a ret variable and simplifies error labels
4) Changes error message to a slightly more descriptive one and gets rid
   of the newline at the end:

Before:
$ virsh cpu-stats tr
error: Failed to virDomainGetCPUStats()

error: Requested operation is not valid: domain is not running

After:
$ tools/virsh cpu-stats tr
error: Failed to retrieve CPU statistics for domain 'tr'
error: Requested operation is not valid: domain is not running