arm64: Rework set_pmu_attr()

By the time kvmtool generates the DTB node for the PMU, the
KVM_ARM_VCPU_PMU_V3 VCPU feature is already set by kvm_cpu__arch_init().

KVM refuses to run a VCPU if the PMU hasn't been initialized. A PMU
cannot be initialized if the interrupt ID hasn't been set by userspace.
As a consequence, kvmtool will get an error if the interrupt ID or if
the PMU has not been initialized:

KVM_RUN failed: Invalid argument

To make debugging easier, exit with an error message as soon as one the
PMU ioctls fails, instead of waiting until the VCPU is first run.

To avoid the repetition of assigning a new kvm_device_attr struct in the
main body of pmu__generate_fdt_nodes(), which hinders readability of the
function, move the struct to set_pmu_attr().

Signed-off-by: Alexandru Elisei <alexandru.elisei@arm.com>