libxl: relax readonly check introduced by XSA-142 fix

libxl: relax readonly check introduced by XSA-142 fix

The fix for XSA-142 is quite a big hammer, rejecting readonly
disk configuration even when the requested backend is known to
support readonly. While it is true that qemu doesn't support
readonly for emulated IDE or AHCI disks

$ /usr/lib/xen/bin/qemu-system-i386 \
 -drive file=/tmp/disk.raw,if=ide,media=disk,format=raw,readonly=on
qemu-system-i386: Can't use a read-only drive

$ /usr/lib/xen/bin/qemu-system-i386 -device ahci,id=ahci0 \
 -drive file=/tmp/disk.raw,if=none,id=ahcidisk-0,format=raw,readonly=on \
 -device ide-hd,bus=ahci0.0,unit=0,drive=ahcidisk-0
qemu-system-i386: -device ide-hd,bus=ahci0.0,unit=0,drive=ahcidisk-0:
Can't use a read-only drive

It does support readonly SCSI disks

$ /usr/lib/xen/bin/qemu-system-i386 \
 -drive file=/tmp/disk.raw,if=scsi,media=disk,format=raw,readonly=on
[ok]

Inside a guest using such a disk, the SCSI kernel driver sees write
protect on

[   7.339232] sd 2:0:1:0: [sdb] Write Protect is on

Also, PV drivers support readonly, but the patch rejects such
configuration even when PV drivers (vdev=xvd*) have been explicitly
specified and creation of an emulated twin is skiped.

This follow-up patch loosens the restriction to reject readonly when
creating an emulated IDE or AHCI disk, but allows it when the backend
is known to support readonly.

Signed-off-by: Jim Fehlig <jfehlig@suse.com>
Acked-by: Stefano Stabellini <stefano.stabellini@eu.citrix.com>
Acked-by: Ian Campbell <ian.campbell@citrix.com>