virStorageSourceParseNBDColonString: Rewrite to match what qemu does

virStorageSourceParseNBDColonString: Rewrite to match what qemu does

Our implementation wasn't quite able to parse everything that qemu does.
This patch rewrites the parser to a code that semantically resembles the
combination of 'nbd_parse_filename' and 'inet_parse' methods in qemu to
be able to parse the strings in an equivalent manner.

The only thing that libvirt doesn't do is to check the lengths of
various components in the nbd string in places where qemu uses constant
size buffers.

The test cases validate that some of the corner cases involving colons
are parsed properly.

https://bugzilla.redhat.com/show_bug.cgi?id=1826652

Signed-off-by: Peter Krempa <pkrempa@redhat.com>
Reviewed-by: Eric Blake <eblake@redhat.com>