virsh: Report errors if arguments of the schedinfo command are incorrect

virsh: Report errors if arguments of the schedinfo command are incorrect

Libvirt's helper API's when called directly don't raise the error so
that virsh remembers it. Subsequent calls to libvirt API's might reset
the error.

In case of schedinfo virDomainFree() in the cleanup section resets the
error when virTypedParameterAssignFromStr() fails.

This patch adds function vshSaveLibvirtError() that can be called after
calling libvirt helper APIs to ensure the error is remembered.