tests: move make_vm_name to QubesTestCase
tests: move make_vm_name to QubesTestCase

This function is often useful also in unit tests, not only integration
tests.

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