KVM: arm64: Make the exposed feature bits in AA64DFR0_EL1 writable from userspace

KVM: arm64: Make the exposed feature bits in AA64DFR0_EL1 writable from userspace

KVM exposes the OS double lock feature bit to Guests but returns
RAZ/WI on Guest OSDLR_EL1 access. This breaks Guest migration between
systems where this feature differ. Add support to make this feature
writable from userspace by setting the mask bit. While at it, set the
mask bits for the exposed WRPs(Number of Watchpoints) as well.
Also update the selftest to cover these fields.

However we still can't make BRPs and CTX_CMPs fields writable, because
as per ARM ARM DDI 0487K.a, section D2.8.3 Breakpoint types and
linking of breakpoints, highest numbered breakpoints(BRPs) must be
context aware breakpoints(CTX_CMPs). KVM does not trap + emulate the
breakpoint registers, and as such cannot support a layout that misaligns
with the underlying hardware.

Reviewed-by: Oliver Upton <oliver.upton@linux.dev>
Signed-off-by: Shameer Kolothum <shameerali.kolothum.thodi@huawei.com>
Link: https://lore.kernel.org/r/20240816132819.34316-1-shameerali.kolothum.thodi@huawei.com
Signed-off-by: Marc Zyngier <maz@kernel.org>