qemuBuildMemoryBackendProps: Respect //memoryBacking/allocation/@mode=immediate

qemuBuildMemoryBackendProps: Respect //memoryBacking/allocation/@mode=immediate

If user specifies immediate memory allocation in the domain XML,
they want QEMU to fully allocate its memory. And if the memory
was allocated using plain '-m' then we would honour it. But, if a
memory backend is used, then we don't set the prealloc attribute
of the backend.

Signed-off-by: Michal Privoznik <mprivozn@redhat.com>
Reviewed-by: Ján Tomko <jtomko@redhat.com>
Reviewed-by: Daniel Henrique Barboza <danielhb413@gmail.com>