libqos: allow zero-size allocations

libqos: allow zero-size allocations

As part of streamlining the AHCI tests interface, it'd be nice
if specying a size of zero could be handled without special branches
and the allocator could handle this special case gracefully.

This lets me use the "ahci_io" macros for non-data commands, too,
which moves me forward towards shepherding all AHCI qtests into
a common set of commands in a unified pipeline.

Signed-off-by: John Snow <jsnow@redhat.com>
Message-id: 1452282920-21550-6-git-send-email-jsnow@redhat.com