qubesd: fix response message header
qubesd: fix response message header

Type is not 16 bit big-endian. Encode it as 8bit code and \x00 as
delimiter explicitly.

QubesOS/qubes-issues#853

woju/qubes-core-admin,marmarek/qubes-core-admin,marmarek/qubes-core-admin,woju/qubes-core-admin,QubesOS/qubes-core-admin,woju/qubes-core-admin,marmarek/qubes-core-admin,woju/qubes-core-admin,QubesOS/qubes-core-admin,QubesOS/qubes-core-admin