(squash?) arm64: check virtual EL2 availability

When trying to run with "--nested" on a kernel that doesn't support it,
at the moment kvmtool will bail out with a not very meaningful error
message.
Use the kernel's capability mechanism to check this before trying to
enable virtual EL2.

TO BE SQUASHED once the KVM_CAP_ARM_EL2 value has been finalised.

Signed-off-by: Andre Przywara <andre.przywara@arm.com>