qemu: fix memory leak of @vporttype during migration.

qemu: fix memory leak of @vporttype during migration.

12 bytes in 1 blocks are definitely lost in loss record 188 of 1,145
at 0x4C2B6CD: malloc (in /usr/lib/valgrind/vgpreload_memcheck-amd64-linux.so)
by 0x5D2CD77: xmlStrndup (in /lib/x86_64-linux-gnu/libxml2.so.2.7.8)
by 0x514E137: virXMLPropString (virxml.c:506)
by 0x234F51: qemuMigrationCookieNetworkXMLParse qemu_migration.c:1001)
by 0x235FF8: qemuMigrationCookieXMLParse (qemu_migration.c:1333)
by 0x236214: qemuMigrationCookieXMLParseStr (qemu_migration.c:1372)
by 0x2365D2: qemuMigrationEatCookie (qemu_migration.c:1456)
by 0x243DBA: qemuMigrationFinish (qemu_migration.c:6381)
by 0x204032: qemuDomainMigrateFinish3 (qemu_driver.c:13228)
by 0x521CCBB: virDomainMigrateFinish3 (libvirt-domain.c:4788)
by 0x1936DE: remoteDispatchDomainMigrateFinish3 (remote.c:4580)
by 0x16DBB1: remoteDispatchDomainMigrateFinish3Helper(remote_dispatch.h:7582)

Signed-off-by: ZhangZijian <zhang.zijian@h3c.com>