conf: Resolve Coverity FORWARD_NULL

conf: Resolve Coverity FORWARD_NULL

The recent changes to perform SCSI device address checks during the
post parse callbacks ran afoul of the Coverity checker since the changes
assumed that the 'xmlopt' parameter to virDomainDeviceDefPostParse
would be non NULL (commit id 'ca2cf74e87'); however, what was missed
is there was an "if (xmlopt &&" check being made, so Coverity believed
that it could be possible for a NULL 'xmlopt'.

Checking the various calling paths seemingly disproves that. If called
from virDomainDeviceDefParse, there were two other possible calls that
would end up dereffing, so that path could not be NULL. If called via
virDomainDefPostParseDeviceIterator via virDomainDefPostParse there
are two callers (virDomainDefParseXML and qemuParseCommandLine)
which deref xmlopt either directly or through another call.

So I'm removing the check for non-NULL xmlopt.