qemu: Assume QEMU_CAPS_DISK_WRITE_CACHE

qemu: Assume QEMU_CAPS_DISK_WRITE_CACHE

Introduced in QEMU's commit of v2.7.0-rc0~32^2~5 the .write-cache
attribute of virtio-blk dvice is always available for all QEMU
versions we support (4.2.0, currently). Therefore, we can assume
the capability is always set and thus doesn't need to be checked
for.

The change in some .args is justified, because the qemuxml2argvdatatest
runs these test caseses with very minimalistic set of capabilities,
that's nowhere near real life scenario.

Signed-off-by: Michal Privoznik <mprivozn@redhat.com>
Reviewed-by: Peter Krempa <pkrempa@redhat.com>
Reviewed-by: Ján Tomko <jtomko@redhat.com>