ArmVirtPkg: move UefiBootManagerLib resolution to ArmVirt.dsc.inc

Recent changes to ShellPkg require a resolution for UefiBootManagerLib
for all platforms in ArmVirtPkg. So move the resolution to the shared
include ArmVirt.dsc.inc.

Contributed-under: TianoCore Contribution Agreement 1.0
Signed-off-by: Ard Biesheuvel <ard.biesheuvel@linaro.org>
Reviewed-by: Ruiyu Ni <ruiyu.ni@intel.com>
Reviewed-by: Laszlo Ersek <lersek@redhat.com>