hw/xen: cleanup sourcesets

xen_ss is added unconditionally to arm_ss and i386_ss (the only
targets that can have CONFIG_XEN enabled) and its contents are gated by
CONFIG_XEN; xen_specific_ss has no condition for its constituent files
but is gated on CONFIG_XEN when its added to specific_ss.

So xen_ss is a duplicate of xen_specific_ss, though defined in a
different way.  Merge the two by eliminating xen_ss.

Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
diff --git a/hw/arm/meson.build b/hw/arm/meson.build
index 11eb911..a6feaf1 100644
--- a/hw/arm/meson.build
+++ b/hw/arm/meson.build
@@ -64,7 +64,6 @@
 arm_ss.add(when: 'CONFIG_FSL_IMX6UL', if_true: files('fsl-imx6ul.c', 'mcimx6ul-evk.c'))
 arm_ss.add(when: 'CONFIG_NRF51_SOC', if_true: files('nrf51_soc.c'))
 arm_ss.add(when: 'CONFIG_XEN', if_true: files('xen_arm.c'))
-arm_ss.add_all(xen_ss)
 
 system_ss.add(when: 'CONFIG_ARM_SMMUV3', if_true: files('smmu-common.c'))
 system_ss.add(when: 'CONFIG_EXYNOS4', if_true: files('exynos4_boards.c'))
diff --git a/hw/i386/meson.build b/hw/i386/meson.build
index ff87906..369c6bf 100644
--- a/hw/i386/meson.build
+++ b/hw/i386/meson.build
@@ -32,6 +32,5 @@
 subdir('xen')
 
 i386_ss.add_all(xenpv_ss)
-i386_ss.add_all(xen_ss)
 
 hw_arch += {'i386': i386_ss}
diff --git a/hw/xen/meson.build b/hw/xen/meson.build
index 277f9f2..d887fa9 100644
--- a/hw/xen/meson.build
+++ b/hw/xen/meson.build
@@ -12,6 +12,10 @@
 ))
 
 xen_specific_ss = ss.source_set()
+xen_specific_ss.add(files(
+  'xen-mapcache.c',
+  'xen-hvm-common.c',
+))
 if have_xen_pci_passthrough
   xen_specific_ss.add(files(
     'xen-host-pci-device.c',
@@ -26,10 +30,3 @@
 endif
 
 specific_ss.add_all(when: ['CONFIG_XEN', xen], if_true: xen_specific_ss)
-
-xen_ss = ss.source_set()
-
-xen_ss.add(when: 'CONFIG_XEN', if_true: files(
-  'xen-mapcache.c',
-  'xen-hvm-common.c',
-))