Description of problem:
Fail to inspect-get-icon for windows guests

Version-Release number of selected component (if applicable):
libguestfs-1.16.10-1.el6.x86_64

How reproducible:
100%

Steps to Reproduce:
1. download a pre-installed Windows guest image
2. Inspect the icon of Windows guests as following:
# guestfish
$ trace 1
$ add Win7-32-hvm..raw
$ run
$ inspect-os
/dev/vda1
$ mount /dev/vda1 /
$ inspect-get-icon /dev/vda1
libguestfs: trace: inspect_get_icon "/dev/vda1"
libguestfs: trace: case_sensitive_path "/etc/favicon.png"
libguestfs: trace: case_sensitive_path = NULL (error)
libguestfs: trace: exists "/etc/favicon.png"
libguestfs: trace: exists = 0
libguestfs: trace: inspect_get_icon = ""

Actual results:
At step2
$ inspect-get-icon /dev/vda1
A zero-length buffer is returned.

Expected results:
At Step2
$ inspect-get-icon /dev/vda1
A buffer containing a PNG image should be returned.

Additional info:
1. In fact, on previous version: libguestfs-1.16.8-1.el6.x86_64
We can get icon from Windows XP 32 bit guest, and Win7 32 bit guest.

In the new vesrion, none of them work.
It seems libguestfs doesn't try the external tools on host to get the os icon even they are installed.
It has:
#if defined(WRESTOOL) && defined(BMPTOPNM) && defined(PNMTOPNG) && \
    defined(PAMCUT)
#define CAN_DO_WINDOWS 1
#endif
but these values (WRESTOOL, BMPTOPNM， PNMTOPNG， PAMCUT) are defined while it's building, so if the build system did have these external tools installed, then the rpm which was built on that host will disable this feature

2. For Win7 64bit guest, it fail even on libguestfs-1.16.8-1.el6.x86_64 as following:
><fs> inspect-get-icon /dev/vda1
libguestfs: trace: list_devices
libguestfs: trace: list_devices = ["/dev/vda"]
libguestfs: trace: list_partitions
libguestfs: trace: list_partitions = ["/dev/vda1"]
libguestfs: trace: lvs
libguestfs: trace: lvs = []
libguestfs: trace: list_dm_devices
libguestfs: trace: list_dm_devices = []
libguestfs: trace: list_md_devices
libguestfs: trace: list_md_devices = []
libguestfs: trace: readdir "/dev"
libguestfs: trace: readdir = NULL (error)
libguestfs: trace: inspect_get_icon "/dev/vda1"
libguestfs: trace: case_sensitive_path "/etc/favicon.png"
libguestfs: trace: case_sensitive_path = NULL (error)
libguestfs: trace: exists "/etc/favicon.png"
libguestfs: trace: exists = 0
libguestfs: trace: case_sensitive_path "/Windows/explorer.exe"
libguestfs: trace: case_sensitive_path = "/Windows/explorer.exe"
libguestfs: trace: filesize "/Windows/explorer.exe"
libguestfs: trace: filesize = 2868224
libguestfs: trace: download "/Windows/explorer.exe" "/dev/fd/3"
libguestfs: trace: download = 0
wrestool: /tmp/libguestfsAb9Zoi/0-explorer: file contains no resources
bmptopnm: Standard Input: read error
pamcut: Error reading magic number from Netpbm image stream.  Most often, this means your input file is empty.
pnmtopng: Error reading magic number from Netpbm image stream.  Most often, this means your input file is empty.
libguestfs: trace: inspect_get_icon = ""