nwfilter: assure virNWFilterSnoop(Eth|Dhcp)Hdr structs don't change size

These two objects are used to access fields in actual ethernet packets
captures with libpcap, so it's essential that they don't change size
for any reason. This patch uses gnulib's verify() macro to make sure
their sizes don't change.
Signed-off-by: NLaine Stump <laine@laine.org>
Reviewed-by: NDaniel P. Berrangé <berrange@redhat.com>

src/nwfilter/nwfilter_dhcpsnoop.c

@@ -195,6 +195,7 @@ struct _virNWFilterSnoopEthHdr {
     uint16_t eh_type;
     uint8_t eh_data[];
 } ATTRIBUTE_PACKED;
+verify(sizeof(struct _virNWFilterSnoopEthHdr) == 14);
 
 typedef struct _virNWFilterSnoopDHCPHdr virNWFilterSnoopDHCPHdr;
 typedef virNWFilterSnoopDHCPHdr *virNWFilterSnoopDHCPHdrPtr;
@@ -216,6 +217,7 @@ struct _virNWFilterSnoopDHCPHdr {
     char      d_file[128];
     uint8_t   d_opts[];
 } ATTRIBUTE_PACKED;
+verify(sizeof(struct _virNWFilterSnoopDHCPHdr) == 236);
 
 /* DHCP options */