qemuMonitorJSONEjectMedia: Don't leak stringified reply

qemuMonitorJSONEjectMedia: Don't leak stringified reply

The return value of virJSONValueToString() should be freed when
no longer needed. This is not the case after 256496e1.

==26902== 138 bytes in 2 blocks are definitely lost in loss record 1,051 of 1,239
==26902==    at 0x4C29F80: malloc (in /usr/lib64/valgrind/vgpreload_memcheck-amd64-linux.so)
==26902==    by 0xAA5F599: strdup (in /lib64/libc-2.21.so)
==26902==    by 0x552BAD9: virStrdup (virstring.c:726)
==26902==    by 0x54F60A7: virJSONValueToString (virjson.c:1790)
==26902==    by 0x1DF6EBB9: qemuMonitorJSONEjectMedia (qemu_monitor_json.c:2225)
==26902==    by 0x1DF57A4C: qemuMonitorEjectMedia (qemu_monitor.c:1985)
==26902==    by 0x1DF1EF2D: qemuDomainChangeEjectableMedia (qemu_hotplug.c:199)
==26902==    by 0x1DF90314: qemuDomainChangeDiskLive (qemu_driver.c:7985)
==26902==    by 0x1DF90476: qemuDomainUpdateDeviceLive (qemu_driver.c:8030)
==26902==    by 0x1DF91ED7: qemuDomainUpdateDeviceFlags (qemu_driver.c:8677)
==26902==    by 0x561785F: virDomainUpdateDeviceFlags (libvirt-domain.c:8559)
==26902==    by 0x134210: remoteDispatchDomainUpdateDeviceFlags (remote_dispatch.h:10966)

==26902== 106 bytes in 1 blocks are definitely lost in loss record 1,033 of 1,239
==26902==    at 0x4C29F80: malloc (in /usr/lib64/valgrind/vgpreload_memcheck-amd64-linux.so)
==26902==    by 0xAA5F599: strdup (in /lib64/libc-2.21.so)
==26902==    by 0x552BAD9: virStrdup (virstring.c:726)
==26902==    by 0x54F60A7: virJSONValueToString (virjson.c:1790)
==26902==    by 0x1DF6EC0C: qemuMonitorJSONEjectMedia (qemu_monitor_json.c:2227)
==26902==    by 0x1DF57A4C: qemuMonitorEjectMedia (qemu_monitor.c:1985)
==26902==    by 0x1DF1EF2D: qemuDomainChangeEjectableMedia (qemu_hotplug.c:199)
==26902==    by 0x1DF90314: qemuDomainChangeDiskLive (qemu_driver.c:7985)
==26902==    by 0x1DF90476: qemuDomainUpdateDeviceLive (qemu_driver.c:8030)
==26902==    by 0x1DF91ED7: qemuDomainUpdateDeviceFlags (qemu_driver.c:8677)
==26902==    by 0x561785F: virDomainUpdateDeviceFlags (libvirt-domain.c:8559)
==26902==    by 0x134210: remoteDispatchDomainUpdateDeviceFlags (remote_dispatch.h:10966)

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