iscsi: retry read, write, flush and unmap on unit attention check conditions

the storage might return a check condition status for various reasons.
(e.g. bus reset, capacity change, thin-provisioning info etc.)

currently all these informative status responses lead to an I/O error
which is populated to the guest. this patch introduces a retry mechanism
to avoid this.

Signed-off-by: Peter Lieven <pl@kamp.de>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
iscsi: add iscsi_truncate support

this patch adds iscsi_truncate which effectively allows for
online resizing of iscsi volumes. for this to work you have
to resize the volume on your storage and then call
block_resize command in qemu which will issue a
readcapacity16 to update the capacity.

v4:
  - factor out complete readcapacity logic into a separate function
  - handle capacity change check condition in readcapacity function
    (this happens if the block_resize cmd is the first iscsi task
    executed after a resize on the storage)

v3:
  - remove switch statement in iscsi_open
  - create separate patch for brdv_drain_all() in bdrv_truncate()

v2:
  - add a general bdrv_drain_all() before bdrv_truncate() to avoid
    in-flight AIOs while the device is truncated
  - since no AIOs are in flight we can use a sync libiscsi call
    to re-read the capacity
  - factor out the readcapacity16 logic as it is redundant
    to iscsi_open() and iscsi_truncate().

Signed-off-by: Peter Lieven <pl@kamp.de>
[allow any type of unit attention check condition in iscsi_readcapacity_sync(),
 as in Message-ID: <51263A2A.6070304@dlhnet.de> - Paolo]
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
iscsi: look for pkg-config file too

Due to library conflicts, Fedora will have to put libiscsi in
/usr/lib/iscsi.  Simplify configuration by using a pkg-config
file.  The Fedora package will distribute one, and the patch
to add it has been sent to upstream libiscsi as well.

Cc: qemu-stable@nongnu.org
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
scsi: do not call scsi_read_data/scsi_write_data for a canceled request

Cc: qemu-stable@nongnu.org
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
scsi-disk: do not complete canceled UNMAP requests

Canceled requests should never be completed, and doing that could cause
accesses to a NULL hba_private field.

Cc: qemu-stable@nongnu.org
Reported-by: Stefan Priebe <s.priebe@profihost.ag>
Tested-by: Stefan Priebe <s.priebe@profihost.ag>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
scsi-disk: handle io_canceled uniformly and correctly

Always check it immediately after calling bdrv_acct_done, and
always do a "goto done" in case the "done" label has to free
some memory---as is the case for scsi_unmap_complete in the
previous patch.

This patch could fix problems that happen when a request is
split into multiple parts, and one of them is canceled.  Then
the next part is fired, but the HBA's cancellation callbacks have
fired already.  Whether this happens or not, depends on how the
block/ driver implements AIO cancellation.  It it does a simple
bdrv_drain_all() or similar, then it will not have a problem.
If it only cancels the given AIOCB, this scenario could happen.

Cc: qemu-stable@nongnu.org
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>