Improve detection of UEFI PI headers.

The _FVH header is the actual fixed header, the GUID identifies the type of
filesystem instead. Since this seems to be the only known GUID valid for
this structure, accept it by itself, and report the size of the volume and
header.

src/binwalk/magic/firmware

@@ -495,7 +495,20 @@
 >35     byte        x           try decryption tool from:
 >35     byte        x           http://download.modem-help.co.uk/mfcs-A/Alcatel/Modems/Misc/
 
-16    string        \xd9\x54\x93\x7a\x68\x04\x4a\x44\x81\xce\x0b\xf6\x17\xd8\x90\xdf    UEFI PI firmware volume
+# http://blogs.phoenix.com/phoenix_technologies_bios/\
+# 2007/02/uefi_pi_10_firm.html
+# (The GUID possibly refer to the official UEFI PI filesystem.)
+# GUID: 7A9354D9-0468-444A-81CE0BF617D890D
+
+16      lelong     0x7a9354d9        UEFI PI Firmware Volume
+>20     leshort    0x0468
+>22     leshort    0x444a
+>24     string     \x81\xce\x0b\xf6\x17\xd8\x90\xdf
+>32     ulequad    x                 \b, volume size: %d
+>40     string     _FVH
+>44     ulequad    >0
+>52     uleshort   x                 \b, header size: %d
+
 
 # http://www.intel.com/content/www/us/en/architecture-and-technology/\
 # unified-extensible-firmware-interface/efi-capsule-specification.html