OvmfPkg: raise PcdShellFileOperationSize to 128KB

OvmfPkg: raise PcdShellFileOperationSize to 128KB

Some UEFI shell commands read and write files in chunks. The chunk size is
given by "PcdShellFileOperationSize", whose default in
"ShellPkg/ShellPkg.dec" is 4KB (0x1000).

The virtio-fs daemon of QEMU advertizes a 128KB maximum buffer size by
default, for the FUSE_WRITE operation.

By raising PcdShellFileOperationSize 32-fold, the number of FUSE write
requests shrinks proportionately, when writing large files. And when a
Virtio Filesystem is not used, a 128KB chunk size is still not
particularly wasteful.

Some ad-hoc measurements on my laptop, using OVMF:

- The time it takes to copy a ~270MB file from a Virtio Filesystem to the
  same Virtio Filesystem improves from ~9 seconds to ~1 second.

- The time it takes to compare two identical ~270MB files on the same
  Virtio Filesystem improves from ~11 seconds to ~3 seconds.

Cc: Ard Biesheuvel <ard.biesheuvel@arm.com>
Cc: Jordan Justen <jordan.l.justen@intel.com>
Cc: Philippe Mathieu-Daudé <philmd@redhat.com>
Ref: https://bugzilla.tianocore.org/show_bug.cgi?id=3125
Signed-off-by: Laszlo Ersek <lersek@redhat.com>
Acked-by: Ard Biesheuvel <ard.biesheuvel@arm.com>
Message-Id: <20210113085453.10168-3-lersek@redhat.com>
Reviewed-by: Philippe Mathieu-Daudé <philmd@redhat.com>