domain_event: Resolve memory leak found by Valgrind

domain_event: Resolve memory leak found by Valgrind

Commit id '4421e257' strdup'd devAlias, but didn't free

Running qemuhotplugtest under valgrind resulted in the following:

==7375== 9 bytes in 1 blocks are definitely lost in loss record 11 of 70
==7375==    at 0x4A0887C: malloc (vg_replace_malloc.c:270)
==7375==    by 0x37C1085D71: strdup (strdup.c:42)
==7375==    by 0x4CBBD5F: virStrdup (virstring.c:554)
==7375==    by 0x4CFF9CB: virDomainEventDeviceRemovedNew (domain_event.c:1174)
==7375==    by 0x427791: qemuDomainRemoveChrDevice (qemu_hotplug.c:2508)
==7375==    by 0x42C65D: qemuDomainDetachChrDevice (qemu_hotplug.c:3357)
==7375==    by 0x41C94F: testQemuHotplug (qemuhotplugtest.c:115)
==7375==    by 0x41D817: virtTestRun (testutils.c:168)
==7375==    by 0x41C400: mymain (qemuhotplugtest.c:322)
==7375==    by 0x41DF3A: virtTestMain (testutils.c:764)
==7375==    by 0x37C1021A04: (below main) (libc-start.c:225)