qemu: block: Introduce helpers for properly testing for 'raw' and 'luks' images

qemu: block: Introduce helpers for properly testing for 'raw' and 'luks' images

Unfortunately a LUKS image to be decrypted by qemu has
VIR_STORAGE_FILE_RAW as format, but has encryption properties populated.

Many places in the code don't check it properly and also don't check
properly whether the image is indeed LUKS to be decrypted by qemu.

Introduce helpers which will simplify this task.

Signed-off-by: Peter Krempa <pkrempa@redhat.com>
Reviewed-by: Ján Tomko <jtomko@redhat.com>
Spellchecked-by: Ján Tomko <jtomko@redhat.com>