KVM: arm-vgic: Support KVM_CREATE_DEVICE for VGIC

Support creating the ARM VGIC device through the KVM_CREATE_DEVICE
ioctl, which can then later be leveraged to use the
KVM_{GET/SET}_DEVICE_ATTR, which is useful both for setting addresses in
a more generic API than the ARM-specific one and is useful for
save/restore of VGIC state.

Adds KVM_CAP_DEVICE_CTRL to ARM capabilities.

Note that we change the check for creating a VGIC from bailing out if
any VCPUs were created, to bailing out if any VCPUs were ever run.  This
is an important distinction that shouldn't break anything, but allows
creating the VGIC after the VCPUs have been created.

Acked-by: Marc Zyngier <marc.zyngier@arm.com>
Signed-off-by: Christoffer Dall <christoffer.dall@linaro.org>