fillQemuCaps: Don't leak machine string

fillQemuCaps: Don't leak machine string

==25251== 5 bytes in 1 blocks are definitely lost in loss record 7 of 81
==25251==    at 0x4C2BEDF: malloc (vg_replace_malloc.c:299)
==25251==    by 0x967E379: strdup (in /lib64/libc-2.25.so)
==25251==    by 0x5366F9F: virStrdup (virstring.c:941)
==25251==    by 0x538BF1D: virDomainCapsNew (domain_capabilities.c:121)
==25251==    by 0x10EACE: test_virDomainCapsFormat (domaincapstest.c:295)
==25251==    by 0x10FBD2: virTestRun (testutils.c:180)
==25251==    by 0x10F192: mymain (domaincapstest.c:457)
==25251==    by 0x111C7F: virTestMain (testutils.c:1119)
==25251==    by 0x10FA3C: main (domaincapstest.c:528)

Signed-off-by: Michal Privoznik <mprivozn@redhat.com>
Reviewed-by: John Ferlan <jferlan@redhat.com>