Merge remote-tracking branch 'remotes/maxreitz/tags/pull-block-2019-06-24' into staging

Block patches:
- The SSH block driver now uses libssh instead of libssh2
- The VMDK block driver gets read-only support for the seSparse
  subformat
- Various fixes

# gpg: Signature made Mon 24 Jun 2019 15:42:56 BST
# gpg:                using RSA key 91BEB60A30DB3E8857D11829F407DB0061D5CF40
# gpg:                issuer "mreitz@redhat.com"
# gpg: Good signature from "Max Reitz <mreitz@redhat.com>" [full]
# Primary key fingerprint: 91BE B60A 30DB 3E88 57D1  1829 F407 DB00 61D5 CF40

* remotes/maxreitz/tags/pull-block-2019-06-24:
  iotests: Fix 205 for concurrent runs
  ssh: switch from libssh2 to libssh
  vmdk: Add read-only support for seSparse snapshots
  vmdk: Reduce the max bound for L1 table size
  vmdk: Fix comment regarding max l1_size coverage
  iotest 134: test cluster-misaligned encrypted write
  blockdev: enable non-root nodes for transaction drive-backup source
  nvme: do not advertise support for unsupported arbitration mechanism

Signed-off-by: Peter Maydell <peter.maydell@linaro.org>