qemuBuildSCSIiSCSIHostdevDrvStr: Don't leak @netsource and @srcprops

qemuBuildSCSIiSCSIHostdevDrvStr: Don't leak @netsource and @srcprops

After 6b770f9a3bdabb1 both @netsource and @srcprops are leaked
because of early return introduced in the commit.

==1812== 644 bytes in 4 blocks are definitely lost in loss record 835 of 885
==1812==    at 0x4C2F12F: realloc (vg_replace_malloc.c:785)
==1812==    by 0x8846393: xmlSaveUriRealloc (in /usr/lib64/libxml2.so.2.9.8)
==1812==    by 0x8846B1C: xmlSaveUri (in /usr/lib64/libxml2.so.2.9.8)
==1812==    by 0x5DDA619: virURIFormat (viruri.c:256)
==1812==    by 0x56E941B: qemuBuildNetworkDriveURI (qemu_command.c:781)
==1812==    by 0x56E979A: qemuBuildNetworkDriveStr (qemu_command.c:859)
==1812==    by 0x56F3A0B: qemuBuildSCSIiSCSIHostdevDrvStr (qemu_command.c:4664)
==1812==    by 0x56F3D1F: qemuBuildSCSIHostdevDrvStr (qemu_command.c:4732)
==1812==    by 0x56F57F7: qemuBuildHostdevCommandLine (qemu_command.c:5337)
==1812==    by 0x570303A: qemuBuildCommandLine (qemu_command.c:10376)
==1812==    by 0x57604EE: qemuProcessCreatePretendCmd (qemu_process.c:6649)
==1812==    by 0x11352A: testCompareXMLToArgv (qemuxml2argvtest.c:566)

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