qemuhotplugtest: Fix mem-leaking testcases

qemuhotplugtest: Fix mem-leaking testcases

While running qemuhotplugtest, it was found that valgrind pointed out
the following memory leak:

==7906== 5 bytes in 1 blocks are definitely lost in loss record 7 of 121
==7906==    at 0x4A069EE: malloc (vg_replace_malloc.c:270)
==7906==    by 0x3E782A754D: xmlStrndup (in /usr/lib64/libxml2.so.2.7.6)
==7906==    by 0x4CDAE03: virDomainDeviceInfoParseXML.isra.32 (domain_conf.c:3685)
==7906==    by 0x4CE3BB9: virDomainNetDefParseXML (domain_conf.c:6707)
==7906==    by 0x4CFBA08: virDomainDefParseXML (domain_conf.c:12235)
==7906==    by 0x4CFBC1E: virDomainDefParseNode (domain_conf.c:13039)
==7906==    by 0x4CFBD95: virDomainDefParse (domain_conf.c:12981)
==7906==    by 0x41FEB4: testQemuHotplug (qemuhotplugtest.c:66)
==7906==    by 0x420F41: virtTestRun (testutils.c:201)
==7906==    by 0x41F287: mymain (qemuhotplugtest.c:422)
==7906==    by 0x4216BD: virtTestMain (testutils.c:784)
==7906==    by 0x3E6CE1ED1C: (below main) (libc-start.c:226)
...and 10 more.

Problem is, since 20745748 we do both, parse <alias/> elements from
XML files and call qemuAssignDeviceAliases(). While generating runtime
info for domain at runtime is just fine in the test, we can parse just
inactive XML and remove all <alias/>-es from the XML files.

Signed-off-by: Michal Privoznik <mprivozn@redhat.com>