KVM: arm64/sve: allow KVM_ARM_SVE_CONFIG_QUERY on vm fd

Since userspace may need to decide on the set of vector lengths for
the guest before setting up a vm, it is onerous to require a vcpu
fd to be available first.  KVM_ARM_SVE_CONFIG_QUERY is not
vcpu-dependent anyway, so this patch wires up KVM_ARM_SVE_CONFIG to
be usable on a vm fd where appropriate.

Subcommands that are vcpu-dependent (currently
KVM_ARM_SVE_CONFIG_SET, KVM_ARM_SVE_CONFIG_GET) will return -EINVAL
if invoked on a vm fd.

Signed-off-by: Dave Martin <Dave.Martin@arm.com>