hw/vfio/pci-quirks: Fix missing ERRP_GUARD() for error_prepend()

As the comment in qapi/error, passing @errp to error_prepend() requires
ERRP_GUARD():

* = Why, when and how to use ERRP_GUARD() =
*
* Without ERRP_GUARD(), use of the @errp parameter is restricted:
...
* - It should not be passed to error_prepend(), error_vprepend() or
*   error_append_hint(), because that doesn't work with &error_fatal.
* ERRP_GUARD() lifts these restrictions.
*
* To use ERRP_GUARD(), add it right at the beginning of the function.
* @errp can then be used without worrying about the argument being
* NULL or &error_fatal.

ERRP_GUARD() could avoid the case when @errp is &error_fatal, the user
can't see this additional information, because exit() happens in
error_setg earlier than information is added [1].

In hw/vfio/pci-quirks.c, there are 2 functions passing @errp to
error_prepend() without ERRP_GUARD():
- vfio_add_nv_gpudirect_cap()
- vfio_add_vmd_shadow_cap()

There are too many possible callers to check the impact of this defect;
it may or may not be harmless. Thus it is necessary to protect their
@errp with ERRP_GUARD().

To avoid the issue like [1] said, add missing ERRP_GUARD() at the
beginning of this function.

[1]: Issue description in the commit message of commit ae7c80a
     ("error: New macro ERRP_GUARD()").

Cc: Alex Williamson <alex.williamson@redhat.com>
Cc: Cédric Le Goater <clg@redhat.com>
Signed-off-by: Zhao Liu <zhao1.liu@intel.com>
Reviewed-by: Cédric Le Goater <clg@redhat.com>
Message-ID: <20240311033822.3142585-23-zhao1.liu@linux.intel.com>
Signed-off-by: Thomas Huth <thuth@redhat.com>

hw/vfio/pci-quirks.c

@@ -1538,6 +1538,7 @@ static bool is_valid_std_cap_offset(uint8_t pos)
 
 static int vfio_add_nv_gpudirect_cap(VFIOPCIDevice *vdev, Error **errp)
 {
+    ERRP_GUARD();
     PCIDevice *pdev = &vdev->pdev;
     int ret, pos;
     bool c8_conflict = false, d4_conflict = false;
@@ -1630,6 +1631,7 @@ static int vfio_add_nv_gpudirect_cap(VFIOPCIDevice *vdev, Error **errp)
 #define VMD_SHADOW_CAP_LEN 24
 static int vfio_add_vmd_shadow_cap(VFIOPCIDevice *vdev, Error **errp)
 {
+    ERRP_GUARD();
     uint8_t membar_phys[16];
     int ret, pos = 0xE8;