qemuDomainObjPrivateFree: Free @masterKey too

qemuDomainObjPrivateFree: Free @masterKey too

This one's a bit more complicated. In qemuProcessPrepareDomain()
a master key for encrypting secret for ciphered disks is created.
This object lives within qemuDomainObjPrivate object. It is freed
in qemuProcessStop(), but if nobody calls it (for instance like
our qemuxml2argvtest does), the key object leaks.

==17078== 32 bytes in 1 blocks are definitely lost in loss record 633 of 707
==17078==    at 0x4C2C070: calloc (vg_replace_malloc.c:623)
==17078==    by 0xAD924DF: virAllocN (viralloc.c:191)
==17078==    by 0x5050BA6: virCryptoGenerateRandom (qemuxml2argvmock.c:166)
==17078==    by 0x453DC8: qemuDomainMasterKeyCreate (qemu_domain.c:678)
==17078==    by 0x47A36B: qemuProcessPrepareDomain (qemu_process.c:4913)
==17078==    by 0x47C728: qemuProcessCreatePretendCmd (qemu_process.c:5542)
==17078==    by 0x433698: testCompareXMLToArgvFiles (qemuxml2argvtest.c:332)
==17078==    by 0x4339AC: testCompareXMLToArgvHelper (qemuxml2argvtest.c:413)
==17078==    by 0x446E7A: virTestRun (testutils.c:179)
==17078==    by 0x445BD9: mymain (qemuxml2argvtest.c:2022)
==17078==    by 0x44886F: virTestMain (testutils.c:969)
==17078==    by 0x445D9B: main (qemuxml2argvtest.c:2036)

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