qemuBuildHostNetStr: use type_sep earlier

qemuBuildHostNetStr: use type_sep earlier

When hotplugging networks with ancient QEMUs not supporting
QEMU_CAPS_NETDEV, we use space instead of a comma as the separator
between the network type and other options.

Except for "user", all the network types pass other options
and use up the first separator by the time we get to the section
that adds the alias (or vlan for QEMUs without CAPS_NETDEV).

Since the alias/vlan is mandatory, convert all preceding code to add
the separator at the end, removing the need to rewrite type_sep for
all types but NET_TYPE_USER.