MAINTAINERS: Add arch/x86/kvm sub-directories to existing KVM/x86 entry

MAINTAINERS: Add arch/x86/kvm sub-directories to existing KVM/x86 entry

A series currently sitting in KVM's queue for 4.21 moves the bulk of
KVM's VMX code to a dedicated VMX sub-directory[1].  As a result,
get_maintainers.pl doesn't get any hits on the newly relocated VMX
files when the script is run with --pattern-depth=1.

Add all arch/x86/kvm sub-directories to the existing MAINTAINERS entry
for KVM/x86 instead of arch/x86/kvm/vmx as other code, e.g. SVM, may
get similar treatment in the near future.

[1] https://patchwork.kernel.org/cover/10710751/

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