Iterating over the usual disks into the pools.
(gdb) p *disk->src->srcpool
$9 = {pool = 0x565040c0a590 "internal", volume = 0x565040c09ad0 "foo", voltype = 0, pooltype = 0, actualtype = 0, mode = 0}
(gdb) p *disk->src->srcpool
$11 = {pool = 0x565040c093d0 "testvg", volume = 0x565040c09650 "guest1", voltype = 0, pooltype = 0, actualtype = 0, mode = 0}

And then e.g. in my case the default connection fails.
31231   virDomainDiskTranslateSourcePool(virDomainDiskDefPtr def)
...
31246       if (!(conn = virGetConnectStorage()))

Due to that it returns -1 and we can't get any extra info from the disk

Inside virGetConnectGeneric I see that it can't get a existing cached connection via
145         conn = virThreadLocalGet(threadPtr);
(gdb) p conn
$2 = (virConnectPtr) 0x0

That was expected, then it probes via geteuid and since virt-aa-helper runs like libvirtd as root decides to use
(gdb) p uri
$7 = 0x55a1b10bcce0 "storage:///system"

Then it tries to open this, the backtrace up to here looks like
#0  virConnectOpenInternal (name=0x55a1b10bcce0 "storage:///system", auth=0x0, flags=0) at ../../../src/libvirt.c:820
#1  0x00007f014f83fbb4 in virConnectOpen (name=name@entry=0x55a1b10bcce0 "storage:///system") at ../../../src/libvirt.c:1076
#2  0x00007f014f83b3dd in virGetConnectGeneric (threadPtr=<optimized out>, name=0x7f014f9081a2 "storage") at ../../../src/driver.c:156
#3  0x00007f014f7069c5 in virDomainDiskTranslateSourcePool (def=0x55a1b1074cc0) at ../../../src/conf/domain_conf.c:31246
#4  0x000055a1b0d1552a in get_files (ctl=0x7ffe043579e0) at ../../../src/security/virt-aa-helper.c:951
#5  vahParseArgv (argv=<optimized out>, argc=<optimized out>, ctl=0x7ffe043579e0) at ../../../src/security/virt-aa-helper.c:1442
#6  main (argc=<optimized out>, argv=<optimized out>) at ../../../src/security/virt-aa-helper.c:1492

With these arguments:
Breakpoint 4, virConnectOpenInternal (name=0x55a1b10bcce0 "storage:///system", auth=0x0, flags=0) at ../../../src/libvirt.c:820

(here is the libvirt.conf load BTW)

It tries via storage driver then:

(gdb) p virConnectDriverTab[0]->hypervisorDriver->name
$20 = 0x7f014b8438e4 "storage"
(gdb) p *(virConnectDriverTab[0]->uriSchemes)
$22 = 0x7f014b8438e4 "storage"

Init goes well and it calls into
(gdb) p virConnectDriverTab[0]->hypervisorDriver->connectOpen
$31 = (virDrvConnectOpen) 0x7f014b830490 <storageConnectOpen>

But that returns -2

(gdb) bt
#0  storageConnectOpen (conn=0x55a1b109f920, auth=0x0, conf=0x55a1b10bcce0, flags=0) at ../../../src/storage/storage_driver.c:397
#1  0x00007f014f83e822 in virConnectOpenInternal (name=<optimized out>, auth=0x0, flags=0) at ../../../src/libvirt.c:1000
#2  0x00007f014f83fbb4 in virConnectOpen (name=name@entry=0x55a1b10af4b0 "storage:///system") at ../../../src/libvirt.c:1076
#3  0x00007f014f83b3dd in virGetConnectGeneric (threadPtr=<optimized out>, name=0x7f014f9081a2 "storage") at ../../../src/driver.c:156
#4  0x00007f014f7069c5 in virDomainDiskTranslateSourcePool (def=0x55a1b1075050) at ../../../src/conf/domain_conf.c:31246
#5  0x000055a1b0d1552a in get_files (ctl=0x7ffe043579e0) at ../../../src/security/virt-aa-helper.c:951
#6  vahParseArgv (argv=<optimized out>, argc=<optimized out>, ctl=0x7ffe043579e0) at ../../../src/security/virt-aa-helper.c:1442
#7  main (argc=<optimized out>, argv=<optimized out>) at ../../../src/security/virt-aa-helper.c:1492


And here the problem is that in the context of virt-aa-helper in 
src/storage/storage_driver.c the driver isn't present.


(gdb) p driver
$32 = (virStorageDriverStatePtr) 0x0

We could be blunt and call storageStateInitialize, but I think that would try to re-setup all the pools. Without further analysis I'm not sure this is safe to call in the virt-aa-helper context.

@Garry - was the removal of the virDriverLoadModule related to this - to keep some original driver context?

In the past it just didn't have storage loaded without this and things failed. Sine this isn't the libvirt daemon which has storage loaded (and a driver we could use) but "just" virt-aa-helper I doubt skipping this load will help, but happy to be convinced otherwise.