Prior to this commit, the qubes-core.service inherited systemd's default
timeout value of 90 seconds. With slow hard disk drives, this caused the
dom0 shut-down sequence to proceed even if some VMs were still not fully
shut down at the time of dom0 shut down.
This commit aims to avoid this issue by setting the service stop timeout to
180 seconds.
Signed-off-by: M. Vefa Bicakci <m.v.b@runbox.com>