Maintainer.txt: Add Kvmtool platform reviewer

Maintainer.txt: Add Kvmtool platform reviewer

Kvmtool is a virtual machine manager that can be used
to launch guest partitions. It additionally emulates
some hardware components e.g. RTC, CFI etc. essentially
providing a virtual platform for a guest operating
system (OS) to run.

A standards-based OS would need UEFI firmware support
for the Kvmtool virtual platform, for which additional
modules are added to ArmVirtPkg.
Adding myself as reviewer for these modules as
advised on mailing list discussion at
- https://edk2.groups.io/g/devel/topic/30915279#30693
- https://edk2.groups.io/g/devel/topic/74200911#59650

Signed-off-by: Sami Mujawar <sami.mujawar@arm.com>
Reviewed-by: Laszlo Ersek <lersek@redhat.com>
Reviewed-by: Ard Biesheuvel <Ard.Biesheuvel@arm.com>