assemble_spice_ticket: ensure variable in interpolated string are correct

assemble_spice_ticket: ensure variable in interpolated string are correct

In older perl the following two where the same:

"$foo::$bar" == "${foo}::${bar}"

But in perl 5, version 28 it's not anymore,

"$foo::$bar" would be equivalent to "${foo::}${bar}", the double
colons are now not used as variable name boundary, so mark that
explicitly in the affected case and surrounding ones preventively

This fixes authentication with spice* related stuff again.

Signed-off-by: Thomas Lamprecht <t.lamprecht@proxmox.com>