Only include loop devices in LVM global_filter

The following tempest test is failing regularly in the gate:
tempest.api.compute.volumes.test_attach_volume.AttachVolumeTestJSON.test_list_get_volume_attachments

The theory behind this fix is that tests are creating/deleting /dev/sdX
devices and LVM ends up attempting to open an already removed device
which causes LVM to temporarily block. Setting the global_filter will
limit the block devices that are used by LVM system components.
Microstack only uses a loopback device for LVM.

Closes-Bug: #1918306
Change-Id: I8cccf7a1b1af2e15106b11023652af23c7715e6f

snap-overlay/snap-openstack.yaml

@@ -77,6 +77,7 @@ setup:
     neutron_ovn_metadata_agent.ini.j2: "{snap_common}/etc/neutron/neutron_ovn_metadata_agent.ini"
     rabbitmq.conf.j2: "{snap_common}/etc/rabbitmq/rabbitmq.config"
     iscsid.conf.j2: "{snap_common}/etc/iscsi/iscsid.conf"
+    lvm.conf.j2: "{snap_common}/etc/lvm/lvm.conf"
 
     # LMA stack templates
     telegraf.conf.j2: "{snap_common}/etc/telegraf/telegraf-microstack.conf"

snapcraft.yaml

@@ -59,6 +59,8 @@ layout:
   # to /etc/rtslib-fb-target.
   /etc/rtslib-fb-target:
     bind: $SNAP_COMMON/etc/target
+  /etc/lvm:
+    bind: $SNAP_COMMON/etc/lvm
 apps:
   # Openstack
   openstack: