qemu: Fix NULL ptr dereference caused by qemuDomainDefFormatBufInternal
qemu: Fix NULL ptr dereference caused by qemuDomainDefFormatBufInternal

qemuDomainDefFormatBufInternal function wasn't testing whether the CPU
was actually defined in the XML and saving such a domain resulted in the
following backtrace:

0 in qemuDomainMakeCPUMigratable (cpu=0x0)
1 in qemuDomainDefFormatBufInternal()
2 in qemuDomainDefFormatXMLInternal()
3 in qemuDomainDefFormatLive()
4 in qemuDomainSaveInternal()
5 in qemuDomainSaveFlags()
6 in qemuDomainSave()
7 in virDomainSave()

Signed-off-by: Erik Skultety <2c14d38fa47c8799f1b9c16280abe27f8edfec6e@redhat.com>
Reviewed-by: Daniel Henrique Barboza <627b089ad62deae9aaf06ccfa2c9056df28927e0@gmail.com>

zippy2/libvirt,jardasgit/libvirt,jfehlig/libvirt,jardasgit/libvirt,fabianfreyer/libvirt,jfehlig/libvirt,jfehlig/libvirt,jfehlig/libvirt,libvirt/libvirt,libvirt/libvirt,olafhering/libvirt,jardasgit/libvirt,nertpinx/libvirt,nertpinx/libvirt,crobinso/libvirt,libvirt/libvirt,fabianfreyer/libvirt,jardasgit/libvirt,olafhering/libvirt,crobinso/libvirt,olafhering/libvirt,nertpinx/libvirt,nertpinx/libvirt,fabianfreyer/libvirt,zippy2/libvirt,jardasgit/libvirt,fabianfreyer/libvirt,olafhering/libvirt,zippy2/libvirt,crobinso/libvirt,fabianfreyer/libvirt,nertpinx/libvirt,crobinso/libvirt,libvirt/libvirt,zippy2/libvirt