KVM: selftests: kvm_vm_elf_load() and elfhdr_get() should close fd

KVM: selftests: kvm_vm_elf_load() and elfhdr_get() should close fd

kvm_vm_elf_load() and elfhdr_get() open one file each, but they
never close the opened file descriptor.  If a test repeatedly
creates and destroys a VM with __vm_create(), which
(directly or indirectly) calls those two functions, the test
might end up getting a open failure with EMFILE.
Fix those two functions to close the file descriptor.

Signed-off-by: Reiji Watanabe <reijiw@google.com>
Reviewed-by: Oliver Upton <oliver.upton@linux.dev>
Reviewed-by: Andrew Jones <andrew.jones@linux.dev>
Reviewed-by: Sean Christopherson <seanjc@google.com>
Link: https://lore.kernel.org/r/20221220170921.2499209-2-reijiw@google.com
Signed-off-by: Sean Christopherson <seanjc@google.com>