KVM: VMX: Rename "find_msr_entry" to "vmx_find_uret_msr"

Rename "find_msr_entry" to scope it to VMX and to associate it with
guest_uret_msrs.  Drop the "entry" so that the function name pairs with
the existing __vmx_find_uret_msr(), which intentionally uses a double
underscore prefix instead of appending "index" or "slot" as those names
are already claimed by other pieces of the user return MSR stack.

No functional change intended.

Signed-off-by: Sean Christopherson <sean.j.christopherson@intel.com>
Message-Id: <20200923180409.32255-13-sean.j.christopherson@intel.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>