qemu_snapshot: use virDomainDiskByName while updating domain def

qemu_snapshot: use virDomainDiskByName while updating domain def

When creating external snapshot this function is called only when the VM
is not running so there is only one definition to care about. However,
it will be used by external snapshot revert code for active and inactive
definition and they may be different if a disk was (un)plugged only for
the active or inactive definition.

The current code would crash so use virDomainDiskByName() to get the
correct disk from the domain definition based on the disk name and make
sure it exists.

Signed-off-by: Pavel Hrdina <phrdina@redhat.com>
Reviewed-by: Peter Krempa <pkrempa@redhat.com>