qemuBuildSevCommandLine: fix buffer leak

qemuBuildSevCommandLine: fix buffer leak

The buffer is not freed anywhere. Nor in the error paths. Also
the usage virCommand with respect to buffer is very odd.

==2504== 1,100 bytes in 1 blocks are definitely lost in loss record 167 of 175
==2504==    at 0x4C2CE3F: malloc (vg_replace_malloc.c:298)
==2504==    by 0x4C2F1BF: realloc (vg_replace_malloc.c:785)
==2504==    by 0x5D32EE2: virReallocN (viralloc.c:245)
==2504==    by 0x5D37278: virBufferGrow (virbuffer.c:150)
==2504==    by 0x5D3783E: virBufferVasprintf (virbuffer.c:408)
==2504==    by 0x5D377A9: virBufferAsprintf (virbuffer.c:381)
==2504==    by 0x57017C1: qemuBuildSevCommandLine (qemu_command.c:9707)
==2504==    by 0x57030F7: qemuBuildCommandLine (qemu_command.c:10324)
==2504==    by 0x575FA48: qemuProcessCreatePretendCmd (qemu_process.c:6644)
==2504==    by 0x11351A: testCompareXMLToArgv (qemuxml2argvtest.c:564)
==2504==    by 0x1392F7: virTestRun (testutils.c:180)
==2504==    by 0x137895: mymain (qemuxml2argvtest.c:2900)

Signed-off-by: Michal Privoznik <mprivozn@redhat.com>
Reviewed-by: Ján Tomko <jtomko@redhat.com>