configure: arm: Update kvmtool UART address

kvmtool commit 45b4968e0de1 ("hw/serial: ARM/arm64: Use MMIO at higher
addresses") changed the UART address from 0x3f8 to 0x1000000. Update the
UART early address accordingly when kvm-unit-tests is configured with
--target=kvmtool.

Users of older kvmtool versions can still enjoy having a working early UART
by configuring kvm-unit-tests with --earlycon=uart,mmio,0x3f8. Note that in
this case --target=kvmtool is still recommended because it enables all
erratas.

Signed-off-by: Alexandru Elisei <alexandru.elisei@arm.com>
Signed-off-by: Andrew Jones <drjones@redhat.com>