Fix memory leak in qemuBuildDriveStr()

Fix memory leak in qemuBuildDriveStr()

This patch fixes memory leaks reported by valgrind on running
qemuxml2argvtest; introduced in commit 0df53f04.

Most of them are of the form:

==24777== 15 bytes in 1 blocks are definitely lost in loss record 39 of 129
==24777==    at 0x4A0887C: malloc (vg_replace_malloc.c:270)
==24777==    by 0x341F485E21: strdup (strdup.c:42)
==24777==    by 0x4CADE5F: virStrdup (virstring.c:554)
==24777==    by 0x4362B6: qemuBuildDriveStr (qemu_command.c:3848)
==24777==    by 0x43EF73: qemuBuildCommandLine (qemu_command.c:8500)
==24777==    by 0x426670: testCompareXMLToArgvHelper (qemuxml2argvtest.c:350)
==24777==    by 0x427C01: virtTestRun (testutils.c:138)
==24777==    by 0x41DDB5: mymain (qemuxml2argvtest.c:658)
==24777==    by 0x4282A2: virtTestMain (testutils.c:593)
==24777==    by 0x341F421A04: (below main) (libc-start.c:225)
==24777==

Signed-off-by: Eric Blake <eblake@redhat.com>