Especially with arm64/aarch64, the default value limits the number of
volume attachments to two usually. And when more than two volumes are to
be attached, it will fail with "No more available PCI slots". There is
no one-size-fits-all value here so let operators override the default
value.
Closes-Bug: #1944214
Change-Id: I9b9565873cbaeb575704b94a25d0a8556ab96292