tests: add tests for 'qvm-prefs'
tests: add tests for 'qvm-prefs'

QubesOS/qubes-issues#1184
QubesOS/qubes-issues#1274
QubesOS/qubes-issues#1273
QubesOS/qubes-issues#1272

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