hyperv: Retry after WMI query fails to find dev

During long stress runs the WMI query that is looking for
the iSCSI device number can incorrectly return no data.
If the query is retried the appropriate data can then be
obtained.

This commit adds a retry loop, calling
basevolumeutils.get_device_number_for_target to avoid this situation.
It also handles the case where the devices list returned in
get_device_number_for_target is empty.  The retry loop is
implemented with new mounted_disk_query_retry_count and
mounted_disk_query_retry_interval configuration options.

Unit tests have been added to check the good and bad paths for
get_mounted_disk_from_lun.

DocImpact
Closes-bug: 1247901
Change-Id: I082c4b1694efcd20cce65293cd330b7a0cf7d470
Merge "hyperv: Retry after WMI query fails to find dev"