Add support for shared sanlock leases

Add support for shared sanlock leases

A sanlock lease can be marked as shared (rather
than exclusive)  using SANLK_RES_SHARED flag. This
adds support for that flag and ensures that in auto
disk mode, any shared disks use shared leases. This
also makes any read-only disks be completely
ignored.

These changes remove the need for the option

  ignore_readonly_and_shared_disks

so that is removed

Signed-off-by: Daniel P. Berrange <berrange@redhat.com>