KVM: VMX: Rename vcpu_vmx's "save_nmsrs" to "nr_active_uret_msrs"

Add "uret" into the name of "save_nmsrs" to explicitly associate it with
the guest_uret_msrs array, and replace "save" with "active" (for lack of
a better word) to better describe what is being tracked.  While "save"
is more or less accurate when viewed as a literal description of the
field, e.g. it holds the number of MSRs that were saved into the array
the last time setup_msrs() was invoked, it can easily be misinterpreted
by the reader, e.g. as meaning the number of MSRs that were saved from
hardware at some point in the past, or as the number of MSRs that need
to be saved at some point in the future, both of which are wrong.

No functional change intended.

Signed-off-by: Sean Christopherson <sean.j.christopherson@intel.com>
Message-Id: <20200923180409.32255-7-sean.j.christopherson@intel.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>