OvmfPkg: make the 4MB flash size the default (again)

OvmfPkg: make the 4MB flash size the default (again)

Xen gained support for the 4MB flash image in Xen commit 0d6968635ce5
("hvmloader: avoid tests when they would clobber used memory",
2017-05-19), which is part of Xen 4.9.0-rc6.

The previously default 2MB can be explicitly selected with

  -D FD_SIZE_2MB

or

  -D FD_SIZE_IN_KB=2048

Cc: Jordan Justen <jordan.l.justen@intel.com>
Suggested-by: Jordan Justen <jordan.l.justen@intel.com>
Contributed-under: TianoCore Contribution Agreement 1.0
Signed-off-by: Laszlo Ersek <lersek@redhat.com>
(cherry picked from commit bba8dfbec3bbc4fba7fa6398ba3cf76593e0725e)
Reviewed-by: Jordan Justen <jordan.l.justen@intel.com>
[lersek@redhat.com: reference Xen commit in commit message]
Signed-off-by: Laszlo Ersek <lersek@redhat.com>