qemu: Use 'def->os.loader->nvram' directly instead of 'priv->pflash1'

qemu: Use 'def->os.loader->nvram' directly instead of 'priv->pflash1'

Since we now have a full virStorageSource for storing the nvram path we
don't need the extra dance of transferring the data into the 'pflash1'
variable which was an intermediary solution to use -blockdev.

For now we keep it functionally identical to the previous impl.

Signed-off-by: Peter Krempa <pkrempa@redhat.com>
Reviewed-by: Ján Tomko <jtomko@redhat.com>
Tested-by: Rohit Kumar <rohit.kumar3@nutanix.com>