qemu: block: Extract code for adding NBD exports to 'qemuBlockExportAddNBD'

qemu: block: Extract code for adding NBD exports to 'qemuBlockExportAddNBD'

Centralize the logic deciding which arguments to use when exporting a
block backend via NBD to a single place so that it can be centrally
fixed in upcoming commits to support the new export method via
'block-export-add'.

Additionally this allows simplification of the caller from migration as
the logic deciding which arguments to use is extracted too.

Signed-off-by: Peter Krempa <pkrempa@redhat.com>
Reviewed-by: Michal Privoznik <mprivozn@redhat.com>