qemu: Don't regenerate NVRAM path if parsed from domain XML

qemu: Don't regenerate NVRAM path if parsed from domain XML

After v8.0.0-466-g08101bde5d we unconditionally regenerate per
domain NVRAM path even though it might have been parsed earlier
from domain XML. The way we do that leads to a memleak:

  43 bytes in 1 blocks are definitely lost in loss record 330 of 682
  at 0x483F7E5: malloc (vg_replace_malloc.c:381)
  by 0x50D5B18: g_malloc (in /usr/lib64/libglib-2.0.so.0.7000.2)
  by 0x50EFA4F: g_strdup (in /usr/lib64/libglib-2.0.so.0.7000.2)
  by 0x49E774E: virXPathString (virxml.c:88)
  by 0x4A3F0E4: virDomainDefParseBootLoaderOptions (domain_conf.c:18226)
  by 0x4A3F49C: virDomainDefParseBootOptions (domain_conf.c:18298)
  by 0x4A448C3: virDomainDefParseXML (domain_conf.c:19598)
  by 0x4A487A1: virDomainDefParseNode (domain_conf.c:20404)
  by 0x117FCF: testCompareXMLToArgv (qemuxml2argvtest.c:726)
  by 0x142124: virTestRun (testutils.c:142)
  by 0x1423D4: virTestRunLog (testutils.c:197)
  by 0x140A76: mymain (qemuxml2argvtest.c:3406)

If we parsed NVRAM path from domain XML we must refrain from
generating new path.

Signed-off-by: Michal Privoznik <mprivozn@redhat.com>
Reviewed-by: Peter Krempa <pkrempa@redhat.com>