xen/MSI: re-expose masking capability

Now that the hypervisor intercepts all config space writes and monitors
changes to the masking flags, this undoes the main effect of the
XSA-129 fix, exposing the masking capability again to guests.

Signed-off-by: Jan Beulich <jbeulich@xxxxxxxx>
---
TBD: We probably need to deal with running on an older hypervisor. I
     can't, however, immediately see a way for qemu to find out.

--- a/hw/xen/xen_pt_config_init.c
+++ b/hw/xen/xen_pt_config_init.c
@@ -1333,7 +1333,7 @@ static XenPTRegInfo xen_pt_emu_reg_msi[]
         .init_val   = 0x0000,
         .res_mask   = 0xFE00,
         .ro_mask    = 0x018E,
-        .emu_mask   = 0x017E,
+        .emu_mask   = 0x007E,
         .init       = xen_pt_msgctrl_reg_init,
         .u.w.read   = xen_pt_word_reg_read,
         .u.w.write  = xen_pt_msgctrl_reg_write,
@@ -1387,8 +1387,8 @@ static XenPTRegInfo xen_pt_emu_reg_msi[]
         .offset     = PCI_MSI_MASK_32,
         .size       = 4,
         .init_val   = 0x00000000,
-        .ro_mask    = 0xFFFFFFFF,
-        .emu_mask   = 0xFFFFFFFF,
+        .ro_mask    = 0x00000000,
+        .emu_mask   = 0x00000000,
         .init       = xen_pt_mask_reg_init,
         .u.dw.read  = xen_pt_long_reg_read,
         .u.dw.write = xen_pt_long_reg_write,
@@ -1398,8 +1398,8 @@ static XenPTRegInfo xen_pt_emu_reg_msi[]
         .offset     = PCI_MSI_MASK_64,
         .size       = 4,
         .init_val   = 0x00000000,
-        .ro_mask    = 0xFFFFFFFF,
-        .emu_mask   = 0xFFFFFFFF,
+        .ro_mask    = 0x00000000,
+        .emu_mask   = 0x00000000,
         .init       = xen_pt_mask_reg_init,
         .u.dw.read  = xen_pt_long_reg_read,
         .u.dw.write = xen_pt_long_reg_write,