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 <eskultet@redhat.com>
Reviewed-by: Daniel Henrique Barboza <danielhb413@gmail.com>