Make virSecurityDeviceLabelDefParseXML into generic device <seclabel> parser.

Make virSecurityDeviceLabelDefParseXML into generic device <seclabel> parser.

This is just code motion, allowing us to reuse the same function to
parse the <seclabel> from character devices too.

However it also fixes a possible segfault in the original code if
VIR_ALLOC_N returns an error and the cleanup code (at the error:
label) tries to iterate over the unallocated array (thanks Michal
Privoznik for spotting this).

Signed-off-by: Richard W.M. Jones <rjones@redhat.com>