KVM: selftests: Fix sign extension bug in get_desc64_base()

KVM: selftests: Fix sign extension bug in get_desc64_base()

The function get_desc64_base() performs a series of bitwise left shifts on
fields of various sizes. More specifically, when performing '<< 24' on
'desc->base2' (which is a u8), 'base2' is promoted to a signed integer
before shifting.

In a scenario where base2 >= 0x80, the shift places a 1 into bit 31,
causing the 32-bit intermediate value to become negative. When this
result is cast to uint64_t or ORed into the return value, sign extension
occurs, corrupting the upper 32 bits of the address (base3).

Example:
Given:
  base0 = 0x5000
  base1 = 0xd6
  base2 = 0xf8
  base3 = 0xfffffe7c

Expected return: 0xfffffe7cf8d65000
Actual return:   0xfffffffff8d65000

Fix this by explicitly casting the fields to 'uint64_t' before shifting
to prevent sign extension.

Signed-off-by: MJ Pooladkhay <mj@pooladkhay.com>
Link: https://patch.msgid.link/20251222174207.107331-1-mj@pooladkhay.com
Signed-off-by: Sean Christopherson <seanjc@google.com>