rpm: depend on linux-stubdom package

Install both stubdom implementations: mini-os one (xen-hvm) and linux
one (xen-hvm-stubdom-linux).

QubesOS/qubes-issues#2185

rpm_spec/core-dom0.spec

@@ -89,6 +89,7 @@ Conflicts:      qubes-gui-dom0 < 1.1.13
 %if x%{?backend_vmm} == xxen
 Requires:       xen-runtime
 Requires:       xen-hvm
+Requires:       xen-hvm-stubdom-linux
 Requires:       libvirt-daemon-xen >= 1.2.20-6
 %endif
 Requires:       createrepo