conf: fix formatting of smartcard devices

conf: fix formatting of smartcard devices

My commit 0c1d863 broke formatting of passthrough smartcard devices:
<smartcard mode='passthrough' type='spicevmc'/>

resulted in invalid XML:
    <smartcard mode='passthrough'>
       type='spicevmc'>
      <address type='ccid' controller='0' slot='0'/>
    </smartcard>

Split out chardev source formatting function into two -
one formatting the attributes and other formatting the subelements.

Reported-by: Cole Robinson <crobinso@redhat.com>
Reviewed-by: John Ferlan <jferlan@redhat.com>