dom0/core: introduce is_guid_running() and use it in is_fully_usable()
dom0/core: introduce is_guid_running() and use it in is_fully_usable()

To improve code reuse, especially to remove direct checking for
"/var/run/qubes/guid_running.{0}" in many places.

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