qemu: correct misspelled 'enospc' option, and only use for werror

qemu: correct misspelled 'enospc' option, and only use for werror

This resolves:

     https://bugzilla.redhat.com/show_bug.cgi?id=730909

When support for setting the qemu disk error policy to "enospc" was
added, it was inadvertently spelled "enospace". This patch corrects
that on the qemu commandline (while retaining the "enospace" spelling
for libvirt's XML).

Also, while examining the qemu source, I found that "enospc" is not
allowed for the read error policy, only for write error policy (makes
sense). Since libvirt currently only has a single error policy
setting, when "enospace" is selected, the read error policy is set to
"ignore".