blkdebug: Add ability to override unmap geometries

Make it easier to simulate various unusual hardware setups (for
example, recent commits 3482b9bc and b8d0a980 affect the Dell
Equallogic iSCSI with its 15M preferred and maximum unmap and
write zero sizing, or b2f95fee deals with the Linux loopback
block device having a max_transfer of 64k), by allowing blkdebug
to wrap any other device with further restrictions on various
alignments.

Signed-off-by: Eric Blake <eblake@redhat.com>
Reviewed-by: Max Reitz <mreitz@redhat.com>
Message-id: 20170429191419.30051-9-eblake@redhat.com
Signed-off-by: Max Reitz <mreitz@redhat.com>
(cherry picked from commit 430b26a8)
* prereq for 81c219ac
Signed-off-by: Michael Roth <mdroth@linux.vnet.ibm.com>