OvmfPkg/README: bump minimum QEMU version to 1.7.1, machine types to 1.7

OvmfPkg/README: bump minimum QEMU version to 1.7.1, machine types to 1.7

Due to switching to the QemuFwCfgAcpiPlatformDxe driver earlier in this
series, require QEMU version 1.7.1 in the "OvmfPkg/README" file, and
require 1.7 or later machine types too.

Cc: Ard Biesheuvel <ardb+tianocore@kernel.org>
Cc: Jordan Justen <jordan.l.justen@intel.com>
Cc: Philippe Mathieu-Daudé <philmd@redhat.com>
Ref: https://bugzilla.tianocore.org/show_bug.cgi?id=2122
Signed-off-by: Laszlo Ersek <lersek@redhat.com>
Message-Id: <20210526201446.12554-6-lersek@redhat.com>
Reviewed-by: Ard Biesheuvel <ardb@kernel.org>
Reviewed-by: Philippe Mathieu-Daudé <philmd@redhat.com>