KVM: arm64: PMU: Add a helper to read a vCPU's PMCR_EL0

KVM: arm64: PMU: Add a helper to read a vCPU's PMCR_EL0

Add a helper to read a vCPU's PMCR_EL0, and use it whenever KVM
reads a vCPU's PMCR_EL0.

Currently, the PMCR_EL0 value is tracked per vCPU. The following
patches will make (only) PMCR_EL0.N track per guest. Having the
new helper will be useful to combine the PMCR_EL0.N field
(tracked per guest) and the other fields (tracked per vCPU)
to provide the value of PMCR_EL0.

No functional change intended.

Reviewed-by: Sebastian Ott <sebott@redhat.com>
Signed-off-by: Reiji Watanabe <reijiw@google.com>
Signed-off-by: Raghavendra Rao Ananta <rananta@google.com>
Reviewed-by: Eric Auger <eric.auger@redhat.com>
Signed-off-by: Marc Zyngier <maz@kernel.org>
Link: https://lore.kernel.org/r/20231020214053.2144305-4-rananta@google.com
Signed-off-by: Oliver Upton <oliver.upton@linux.dev>