arm: Fail early if KVM_CAP_ARM_PMU_V3 is not supported

pmu__generate_fdt_nodes() checks if the host has support for PMU in a guest
and prints a warning if that's not the case. However, this check is too
late because the function is called after the VCPU has been created, and
VCPU creation fails if KVM_CAP_ARM_PMU_V3 is not available with a rather
unhelpful error:

$ ./vm run -c1 -m64 -f selftest.flat --pmu
  # lkvm run --firmware selftest.flat -m 64 -c 1 --name guest-1039
  Info: Placing fdt at 0x80200000 - 0x80210000
  Fatal: Unable to initialise vcpu

Move the check for KVM_CAP_ARM_PMU_V3 to kvm_cpu__arch_init() before the
VCPU is created so the user can get a more useful error message. This
also matches the behaviour of KVM_CAP_ARM_EL1_32BIT.

Signed-off-by: Alexandru Elisei <alexandru.elisei@arm.com>
Link: https://lore.kernel.org/r/20210415131725.105675-1-alexandru.elisei@arm.com


Signed-off-by: Will Deacon <will@kernel.org>