qemu: Fix crash hot plugging luks volume

qemu: Fix crash hot plugging luks volume

https://bugzilla.redhat.com/show_bug.cgi?id=1367259

Crash occurs because 'secrets' is being dereferenced in call:

        if (qemuDomainSecretSetup(conn, priv, secinfo, disk->info.alias,
                                  VIR_SECRET_USAGE_TYPE_VOLUME, NULL,
                                  &src->encryption->secrets[0]->seclookupdef,
                                  true) < 0)

(gdb) p *src->encryption
$1 = {format = 2, nsecrets = 0, secrets = 0x0, encinfo = {cipher_size = 0,
    cipher_name = 0x0, cipher_mode = 0x0, cipher_hash = 0x0, ivgen_name = 0x0,
    ivgen_hash = 0x0}}
(gdb) bt
    priv=priv@entry=0x7fffc03be160, disk=disk@entry=0x7fffb4002ae0)
    at qemu/qemu_domain.c:1087
    disk=0x7fffb4002ae0, vm=0x7fffc03a2580, driver=0x7fffc02ca390,
    conn=0x7fffb00009a0) at qemu/qemu_hotplug.c:355

Upon entry to qemuDomainAttachVirtioDiskDevice, src->encryption points
at a valid 'secret' buffer w/ nsecrets == 1; however, the call to
qemuDomainDetermineDiskChain will call virStorageFileGetMetadata
and eventually virStorageFileGetMetadataInternal where the src->encryption
was overwritten when probing the volume.

Commit id 'a48c7141' added code to virStorageFileGetMetadataInternal
to determine if the disk/volume would use/need encryption and allocated
a meta->encryption. This overwrote an existing encryption buffer
already provided by the XML

This patch adds a check for meta->encryption already present before
just allocating and overwriting an existing buffer. It then checks the
existing encryption data to ensure the XML provided format for the
disk matches the expected format read from the disk and errors if there
is a mismatch.