Make QEMU consistently report write caching support for emulated IDE

Make QEMU consistently report write caching support for emulated IDE
drives to fix a hang during SLES 9 HVM guest installation.

Without this, the SLES 9 installer kernels (32 and 64 bit) were
getting inconsistent information from QEMU as to whether the
(emulated) IDE drives support write caching (which they do).  So part
of the kernel thought write caching was enabled (and enabled the usage
of barrier writes) and part of it didn't, which triggered a bug in
which the same barrier write is submitted over and over again ...

Fixed by setting another bit in the WIN_IDENTIFY (IDE drive "identify"
command) response to indicate we really, truly support write caching.

Signed-off-by: David Lively <dlively@virtualiron.com>
Signed-off-by: Ben Guthro <bguthro@virtualiron.com>