vbox: fix SEGV during dumpxml of a serial port

vbox: fix SEGV during dumpxml of a serial port

commit 77a12987a48 changed the "virDomainChrSourceDef source" inside
virDomainChrDef to "virDomainChrSourceDefPtr source", and started
allocating source inside virDomainChrDefNew(), but vboxDumpSerial()
was allocating a virDomainChrDef with a simple VIR_ALLOC() (i.e. never
calling virDomainChrDefNew()), so source was never initialized,
leading to a SEGV any time a serial port was present. The same problem
was created in vboxDumpParallel().

This patch changes vboxDumpSerial() and vboxDumpParallel() to use
virDomainChrDefNew() instead of VIR_ALLOC(), and changes both of those
functions to return an error if virDomainChrDef() (or any other
allocation) fails.

This resolves: https://bugzilla.redhat.com/1536649