Depend on edk2-ovmf-xen instead of bundling OVMF

This fixes two bugs:

1. The bundled OVMF didn't support Xen, so UEFI guests failed to boot.
2. The bundled OVMF didn't include the license text alongside it.

Fixes: QubesOS/qubes-issues#8625

xen.spec.in

@@ -10,7 +10,7 @@
 %define build_qemutrad %{?_with_qemutrad: 1} %{?!_with_qemutrad: 0}
 # build with ovmf from edk2-ovmf unless rpmbuild was run with --without ovmf
 %define build_ovmf %{?_without_ovmf: 0} %{?!_without_ovmf: 1}
-# set to 0 for archs that don't use qemu or ovmf (reduces build dependencies)
+# set to 0 for archs that don't use qemu or ovmf (reduces dependencies)
 %ifnarch x86_64 %{ix86}
 %define build_qemutrad 0
 %define build_ovmf 0
@@ -234,7 +234,7 @@ BuildRequires: systemd-devel
 BuildRequires: libfdt-devel
 %endif
 %if %build_ovmf
-BuildRequires: edk2-ovmf
+Requires: edk2-ovmf-xen
 %endif
 %if %build_hyp
 BuildRequires: bison flex
@@ -411,7 +411,7 @@ CONFIG_EXTRA="--enable-qemu-traditional"
 CONFIG_EXTRA=""
 %endif
 %if %build_ovmf
-CONFIG_EXTRA="$CONFIG_EXTRA --with-system-ovmf=%{_libexecdir}/%{name}/boot/ovmf.bin"
+CONFIG_EXTRA="$CONFIG_EXTRA --with-system-ovmf=%{_datadir}/edk2/xen/OVMF.fd"
 %endif
 %ifnarch armv7hl aarch64
 CONFIG_EXTRA="$CONFIG_EXTRA --with-system-ipxe=/usr/share/ipxe/10ec8139.rom"
@@ -569,10 +569,6 @@ rm -f %{buildroot}/%{_libdir}/efi/xen.efi
 rm -rf %{buildroot}/%{_unitdir}/oxenstored.service
 %endif
 
-%if %build_ovmf
-cat /usr/share/OVMF/OVMF_{VARS,CODE}.fd >%{buildroot}%{_libexecdir}/%{name}/boot/ovmf.bin
-%endif
-
 ############ fixup files in /etc ############
 
 # logrotate
@@ -897,9 +893,6 @@ fi
 %{_libexecdir}/%{name}/boot/xen-shim
 /usr/lib/debug%{_libexecdir}/xen/boot/xen-shim-syms
 %endif
-%if %build_ovmf
-%{_libexecdir}/xen/boot/ovmf.bin
-%endif
 %if %build_stubdom
 %if %build_qemutrad
 %{_libexecdir}/xen/boot/ioemu-stubdom.gz