conf: Remove virDomainResctrlAppend and introduce virDomainResctrlNew

conf: Remove virDomainResctrlAppend and introduce virDomainResctrlNew

Introduced virDomainResctrlNew to do the most part of virDomainResctrlAppend
and move the operation of appending resctrl to @def->resctrls out of
function.

Rather than rely on virDomainResctrlAppend to perform the allocation, move
the onus to the caller and make use of virBitmapNewCopy for @vcpus and
virObjectRef for @alloc, thus removing the need to set each to NULL after the
call.

Signed-off-by: Wang Huaqiang <huaqiang.wang@intel.com>
Reviewed-by: John Ferlan <jferlan@redhat.com>