[libvirt] [PATCH] Allow changing ejectable device in domains using default qemu emulator

When a qemu domain is launched but an emulator is not explicitly defined, the default emulator is used. Attempts to attach ejectable devices to a domains using the default emulator fail with the error: "Error Connecting CDROM: internal error\nCannot determine QEMU argv syntax (null)". The following patch is based on similar code in qemudStartVMDaemon() and qemudBuildCommandLine(). I've tested it and it works. Honestly, though, I think this patch is just a band-aid. I'd like to supply a better patch, but I don't have enough experience to judge certain side effects. Instead of requiring functions to determine the emulator that will be used to launch the next qemu domain, qemudStartVMDaemon() should record the emulator that was used to start the current domain. It should also record the emulator's capabilities. This would make it unnecessary to call virDomainDefDefaultEmulator() in qemudDomainChangeEjectableMedia(). It would also make it possible to remove the virDomainDefDefaultEmulator() call from qemudBuildCommandLine(). However, testCompareXMLToArgvFiles() will fail because qemudBuildCommandLine() needs to know vm->def->emulator and virDomainDefParseFile() doesn't set vm->def->emulator to a default value. I don't know if this is a real problem, or if it just means the test needs to be updated. In addition, if we simply set vm->dev->emulator = strdup(virDomainDefDefaultEmulator()) in qemudStartVMDaemon() then virDomainDefFormat() will dump <emulator>...</emulator> for domains that did't actually have it explicitly defined. I don't know if this is a bug or improvement, but it is a change in behavior.