qemuBuildChrChardevStr: Don't leak @charAlias

qemuBuildChrChardevStr: Don't leak @charAlias

==12618== 110 bytes in 10 blocks are definitely lost in loss record 269 of 295
==12618==    at 0x4C2AE5F: malloc (vg_replace_malloc.c:297)
==12618==    by 0x1CFC6DD7: vasprintf (vasprintf.c:73)
==12618==    by 0x1912B2FC: virVasprintfInternal (virstring.c:551)
==12618==    by 0x1912B411: virAsprintfInternal (virstring.c:572)
==12618==    by 0x50B1FF: qemuAliasChardevFromDevAlias (qemu_alias.c:638)
==12618==    by 0x518CCE: qemuBuildChrChardevStr (qemu_command.c:4973)
==12618==    by 0x522DA0: qemuBuildShmemBackendChrStr (qemu_command.c:8674)
==12618==    by 0x523209: qemuBuildShmemCommandLine (qemu_command.c:8789)
==12618==    by 0x526135: qemuBuildCommandLine (qemu_command.c:9843)
==12618==    by 0x48B4BA: qemuProcessCreatePretendCmd (qemu_process.c:5897)
==12618==    by 0x4378C9: testCompareXMLToArgv (qemuxml2argvtest.c:498)
==12618==    by 0x44D5A6: virTestRun (testutils.c:180)

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