OvmfPkg/OvmfXen: make "PcdPciDisableBusEnumeration" Fixed-at-Build

OvmfPkg/OvmfXen: make "PcdPciDisableBusEnumeration" Fixed-at-Build

The OvmfXen platform specifies the dynamic access method for
"PcdPciDisableBusEnumeration" needlessly.

After the DSC file sets the PCD to TRUE by default, the InitializeXen()
function in XenPlatformPei superfluously sets the PCD to TRUE again. There
are no other writes to the PCD in the platform.

Make the PCD Fixed-At-Build, and remove the access (in fact, the whole
InitializeXen() function) from XenPlatformPei.

Cc: Anthony Perard <anthony.perard@citrix.com>
Cc: Ard Biesheuvel <ardb+tianocore@kernel.org>
Cc: Jordan Justen <jordan.l.justen@intel.com>
Cc: Julien Grall <julien@xen.org>
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: <20210526201446.12554-20-lersek@redhat.com>
Reviewed-by: Ard Biesheuvel <ardb@kernel.org>
Reviewed-by: Philippe Mathieu-Daudé <philmd@redhat.com>