qemu: validate: Allow <transient/> disks

qemu: validate: Allow <transient/> disks

Extract the validation of transient disk option. We support transient
disks in qemu under the following conditions:

 - -blockdev is used
 - the disk source is a local file
 - the disk type is 'disk'
 - the disk is not readonly

Signed-off-by: Masayoshi Mizuma <m.mizuma@jp.fujitsu.com>
Signed-off-by: Peter Krempa <pkrempa@redhat.com>
Tested-by: Masayoshi Mizuma <m.mizuma@jp.fujitsu.com>
Reviewed-by: Ján Tomko <jtomko@redhat.com>
Tested-by: Ján Tomko <jtomko@redhat.com>